Implementing a percentage-based commission for NFT sales

NFT
1

Hey guys, I’m stuck on my NFT marketplace project. I’ve got most of it working, but I’m having trouble with the commission system.

I want to add a 2.5% fee on every sale. Right now, I’m using a fixed fee of 0.1 ETH. Here’s what I’ve tried:

function sellNFT(address nftContract, uint256 itemId, uint256 fee) public payable {
    uint256 cost = listedItems[itemId].price;
    uint256 nftId = listedItems[itemId].tokenId;
    
    require(msg.value == cost, "Wrong price sent");
    
    listedItems[itemId].seller.transfer(msg.value);
    IERC721(nftContract).transferFrom(address(this), msg.sender, nftId);
    listedItems[itemId].owner = payable(msg.sender);
    listedItems[itemId].sold = true;
    soldItems.increment();
    payable(owner).transfer(fee);
}

My JS code calculates the fee like this:

const price = ethers.utils.parseUnits(nft.price.toString(), "ether");
const fee = +((2.5 / 100) * nft.price);
let commission = fee.toString();

let tx = await contract.sellNFT(nftContract, nft.itemId, commission, {
    value: price,
});

But I keep getting an error saying the transaction failed. Any ideas what I’m doing wrong? Thanks!

2

yo, i’ve run into this before. the issue is probably in how ur handling the commission. instead of passing it as a parameter, calculate it in the contract.

try something like this in ur contract:

function sellNFT(address nftContract, uint256 itemId) public payable {
    uint256 price = listedItems[itemId].price;
    uint256 commission = price * 25 / 1000; // 2.5% fee
    require(msg.value == price + commission, "Wrong amount sent");
    // rest of ur logic here
}

this should fix it. lmk if u need more help!

3

hey there ethan_witty! i’ve been tinkering with nft marketplaces too and ran into similar commission headaches. have you considered calculating the fee inside the contract instead of passing it as a parameter? that way you can make sure everything adds up correctly.

here’s a quick idea:

function sellNFT(address nftContract, uint256 itemId) public payable {
    uint256 price = listedItems[itemId].price;
    uint256 commission = (price * 25) / 1000; // 2.5% commission
    require(msg.value >= price + commission, "not enough eth sent");
    
    // transfer price to seller, commission to contract owner
    listedItems[itemId].seller.transfer(price);
    payable(owner).transfer(commission);
    
    // rest of your logic here...
}

then in your js, you’d just need to send the total amount:

const totalCost = ethers.utils.parseUnits((nft.price * 1.025).toString(), "ether");
await contract.sellNFT(nftContract, nft.itemId, { value: totalCost });

what do you think? this approach should handle the commission calculation more reliably. let me know if you try it out!

4

I’ve dealt with a similar issue in my NFT marketplace project. The problem is likely in how you’re handling the commission calculation and transfer. Instead of passing the fee as a parameter, calculate it within the smart contract itself. This ensures consistency and prevents potential manipulation.

Try modifying your sellNFT function like this:

function sellNFT(address nftContract, uint256 itemId) public payable {
    uint256 price = listedItems[itemId].price;
    uint256 commission = price * 25 / 1000; // 2.5% fee
    require(msg.value == price + commission, "Incorrect amount sent");

    listedItems[itemId].seller.transfer(price);
    payable(owner).transfer(commission);
    // Rest of your function logic...
}

Then in your JS, send the total amount:

const totalAmount = ethers.utils.parseUnits((nft.price * 1.025).toString(), "ether");
let tx = await contract.sellNFT(nftContract, nft.itemId, { value: totalAmount });

This approach should resolve your issue and implement the percentage-based commission correctly.