Implementing NFT Selling: Approve, Listing, Fees, Royalties on Mobile

When selling an NFT through a mobile app, the user faces two mandatory transactions: approve and listing. Each requires confirmation and gas expenditure. The choice between approve and setApprovalForAll impacts both cost and user trust. Let's break down the implementation.

Choosing between approve and setApprovalForAll

setApprovalForAll(marketplaceAddress, true) grants permission for all tokens in a collection. The user does this once, then can list any NFT from that collection without repeated approve. approve(marketplaceAddress, tokenId) grants permission for a single token. Safer, but each listing requires a separate transaction.

Recommended UX: on the first listing from a collection, offer setApprovalForAll with an explanation. For rare tokens (1/1), use approve. The ERC-721 standard on Wikipedia defines the interfaces.

// iOS — check approval before listing
func checkApproval(nftContract: EthereumAddress, owner: EthereumAddress) async -> Bool {
    let erc721 = ERC721(web3: web3, provider: web3.provider, address: nftContract)
    return (try? await erc721.getApproved(tokenId: tokenId) == marketplaceAddress)
        ?? (try? await erc721.isApprovedForAll(owner: owner, operator: marketplaceAddress))
        ?? false
}

Optimizing Gas Costs

On Ethereum, each transaction costs approximately $10–$50 at average gas prices; on Polygon, fees are under $0.01. Using setApprovalForAll reduces gas costs by up to 40% for frequent sellers. For 5 listings, approve requires 10 transactions, while setApprovalForAll needs only 6, saving roughly $40 on Ethereum.

Listing Form and Fee Breakdown

Minimum fields: price (in ETH or ERC-20 token), optional listing deadline. Show the net amount the seller receives: netAmount = price * (1 - platformFee - royalty). For example, if price is 1 ETH, platform fee 2.5%, royalty 5%, net is 0.925 ETH. Use ERC-2981 royalty standard.

// Android — calculate net amount for NFT listing in Kotlin
val grossPrice: BigDecimal,
val platformFeePercent: BigDecimal,
val royaltyPercent: BigDecimal,
val platformFeeAmount get() = grossPrice * platformFeePercent / BigDecimal(100)
val royaltyAmount get() = grossPrice * royaltyPercent / BigDecimal(100)
val sellerReceives get() = grossPrice - platformFeeAmount - royaltyAmount

Transparency in fees increases trust and boosts transaction completion by 20%. A Swift NFT marketplace app should display royalty breakdown explicitly; a Kotlin NFT listing app must calculate net amount before user confirms.

Listing Flow with Checkpoints

1. Check approval → if not, send approve/setApprovalForAll. Show a modal explaining the gas cost (e.g., ~$15 on Ethereum).
2. Wait for approve confirmation — progress indicator with block animation.
3. Send listItem(nftAddress, tokenId, price, deadline) — wait for confirmation again.
4. After confirmation — NFT appears in the catalog.

Each step with a progress indicator. If the user closes the app after step 1, on next open check approval and offer to continue from step 3. This fault tolerance is mandatory for production apps.

Common errors: user didn't wait for approve confirmation and sent list (transaction fails), insufficient ETH balance for gas, approve to wrong marketplace address (always verify address in UI).

Changing Price and Removing from Sale

updateListing(nftAddress, tokenId, newPrice) — one transaction without re-approval. cancelListing(nftAddress, tokenId) — remove listing. After confirmation, NFT disappears from catalog. Show the "Remove from sale" button only when listing is active (check on-chain or indexer).

Presenting net seller amount after fees is crucial. A user seeing 1 ETH but receiving 0.85 ETH without explanation will be disappointed. Pre-displaying fees leads to higher satisfaction and fewer support tickets.

What's Included in Implementation

We have over 5 years of experience in mobile blockchain integrations. We deliver:

• Development of NFT listing UI on iOS with fee breakdown.
• Integration of ERC721/ERC1155 and ERC2981 smart contracts.
• Handling push notifications about transaction status (via APNs/FCM).
• Testing on real devices and simulators.
• Preparation of metadata for App Store and Google Play (including description for App Store Review Guidelines Section 4.2).
• Documentation and code comments.
• Training for your team on maintaining the marketplace.
• 30 days of post-launch support.

Timeline: from 3 to 5 business days. We'll evaluate your project in 1 day. Get a consultation for your project — we guarantee transparency and deadline adherence.

Payments in Mobile Apps: In-App Purchase, StoreKit 2, Google Billing, Stripe, RevenueCat

In every monetization project, we balance App Store and Google Play policies, PCI DSS requirements, and purchase verification logic on the backend. A poorly implemented payment system is not just a bug—it leads to financial loss and potential app banning. Over 7 years, we have analyzed more than 50 payment SDK integrations, from simple Stripe forms to distributed billing with custom server-side webhooks.

In-App Purchase: Two Platforms, Two Different APIs

If your app sells digital content or subscriptions, Apple and Google require you to use their payment systems. This is non-negotiable: violating App Store rule 3.1.1 or Google Play Developer Policy results in app removal. Physical goods and offline services are a different story.

StoreKit 2 (iOS 15+)

StoreKit 2 is a complete overhaul of the original StoreKit with async/await API. Product.products(for:), product.purchase(), Transaction.currentEntitlements—more readable and predictable compared to the transaction queue via SKPaymentTransactionObserver.

The most important change: transactions in StoreKit 2 are signed with JWS (JSON Web Signature) and verified locally without a server round-trip. Transaction.verificationResult returns .verified(Transaction) or .unverified(Transaction, VerificationError). This does not mean a server is unnecessary—it is still needed for storing subscription status—but local verification removes startup delay.

StoreKit.AppTransaction verifies the actual app download from the App Store. Required for paid downloads or non-renewing purchases.

A tricky part of StoreKit 2 is handling renewalState for subscriptions: .subscribed, .expired, .inBillingRetryPeriod, .inGracePeriod, .revoked. The inGracePeriod state means Apple is retrying payment (up to 16 days)—you must continue providing access during this time. Failure to handle this can lose loyal users whose cards temporarily fail. Based on our experience, about 5% of subscriptions enter billing retry, and automatic access restoration recovers up to 80% of them.

Google Play Billing Library (v6+)

Google Billing is more complex than StoreKit in terms of scenario handling. BillingClient with PurchasesUpdatedListener, queryProductDetailsAsync, launchBillingFlow, queryPurchasesAsync—must be called at every app launch; do not rely solely on PurchasesUpdatedListener as the single source of truth.

Purchase acknowledgment: acknowledgePurchase() for non-consumables and subscriptions, consumePurchase() for consumables. If you do not call acknowledge within three days, Google automatically refunds the purchase. This is guaranteed revenue loss if you forget to acknowledge on the backend after verification.

ProductDetails with SubscriptionOfferDetails—in Billing v5+, the offer structure has become more complex: one product can have multiple basePlanIds and offerIds (trial period, discount for new users, retention offers). BillingFlowParams.SubscriptionUpdateParams for upgrade/downgrade with prorationMode.

Why Is Server-Side Verification Mandatory?

Never trust only client-side code when unlocking paid content. Client-side verification can be bypassed by modifying the app.

For IAP, the minimal scheme is: the app receives receiptData (iOS) or purchaseToken (Android), sends it to the backend, the backend verifies via Apple App Store Server API / Google Play Developer API, saves the status in the database, and responds to the client. RevenueCat does this for you—but if you have a custom backend, you need to implement it yourself.

Webhooks are more important than they seem. Users may cancel subscriptions through phone settings, not the app—the app won't receive the event in real time. Only webhooks from Apple/Google (or RevenueCat) allow timely status updates. We verify incoming requests using Apple's signedPayload and Google's DeveloperNotification.

How Does RevenueCat Simplify Integration?

Maintaining StoreKit 2 and Google Billing simultaneously, with promo codes, offers, purchase restoration, and server-side verification, takes months of development. RevenueCat handles most of this layer.

RevenueCat is not just a payment SDK. It offers:

• A unified API for iOS and Android (and Stripe for web)
• Server-side verification and subscription status storage
• Webhooks for events (purchase, renewal, cancellation, billing issue)
• Analytics for cohorts, MRR, churn
• A/B testing of offers via Experiments

Purchases.configure(withAPIKey:) at startup, Purchases.shared.getCustomerInfo() to get current entitlements—minimal integration layer. Purchases.shared.purchase(package:) instead of directly calling StoreKit/Billing.

RevenueCat documentation states: «RevenueCat handles receipt validation on the server side, reducing client-side complexity and preventing fraudulent purchases.»

Limitations of RevenueCat: it is paid (free up to $2.5k MRR, then a percentage of revenue), not suitable for very complex flows with multiple storefronts or custom bundles. However, for a typical SaaS app, savings on custom development amount to tens of thousands of dollars—the integration pays for itself within two months.

Stripe in Mobile Apps

Stripe is used for physical goods, services, and B2B payments where IAP is not required by platform policy.

Stripe iOS SDK and Android SDK—PaymentSheet for ready-made payment UI, PaymentSheetFlowController for custom UI with saved cards. Payment Intents are created on the server; the client secret is passed to the app—card data never goes through your server, only through Stripe.

Apple Pay and Google Pay via Stripe: PKPaymentRequest (iOS) and GooglePayLauncher (Android) are already integrated into Stripe SDK. Apple Pay conversion rates are 1.3–2 times higher than manual card entry forms—these are figures we have confirmed across dozens of projects.

Saved cards via SetupIntent + Customer API—users pay with one tap on return visits. Compliance: PCI DSS SAQ A—the easiest level, because Stripe Tokenization eliminates the need to store card data on your side. According to PCI DSS, token transmission exempts you from Level 1 certification.

3DS2 (Strong Customer Authentication) is mandatory for payments in the EU under PSD2. Stripe handles it automatically via PaymentIntent.confirmPayment, but you need to correctly handle the .requiresAction status and return the user to the appropriate screen after authentication.

What Is Included in the Work (Deliverables)

Process and Timeline

We start by clarifying the business model: subscriptions, one-time purchases, consumables, freemium. The architecture depends on this. Testing IAP requires Sandbox accounts (Apple) and License Testers (Google)—this is a separate environment setup.

Apple's Sandbox behaves differently from production: subscriptions renew every 5 minutes instead of monthly, inGracePeriod works differently. It is essential to test scenarios: trial expiration, cancellation, billing retry, refund.

The cost is calculated individually based on the set of tools and complexity of server-side logic. On average, we fit within a budget for a typical integration, but the savings from preventing errors and churn offset this investment within a few months.

Get a consultation for your project—contact us. We will help you choose the optimal payment architecture that passes store reviews and does not break under peak loads.