Solidity - Sell function not working

Question

I have my contract and people can buy my tokens via web3 and MetaMask. Now when I try to execute sell() function throw an exception and always Etherscan says Failed.

I set to sell and buy prices on my contract.

Here is my sell function:

/// @notice Sell `amount` tokens to contract
/// @param amount amount of tokens to be sold
function sell(uint256 amount) public {
    require(address(this).balance >= amount * sellPrice);      // checks if the contract has enough ether to buy
    _transfer(msg.sender, this, amount);              // makes the transfers
    msg.sender.transfer(amount * sellPrice);          // sends ether to the seller. It's important to do this last to avoid recursion attacks
}

I don't know what more I have to do. Does someone have any idea to help me?

UPDATE: Full code: https://pastebin.com/eBYC77GV.

UPDATE2: Etherscan Report: https://etherscan.io/tx/0x1213ca9540b8b7c0bd34f09dac906906772416a31e3b01559d0c0a3c05582a19

Thanks.

Answer 1

If your sell price value is scaled up ie amount * (sellPrice * (10**18)) then you'll have a massively large value, so you'll need to divide the result by 10**18. Say for example sellPrice is meant to be 100 tokens in units of wei, tha would be 100 * (10**18). If you pass in a value for amount that is also in units of wei, you'll have an even larger number:

(amount * (10**18)) * (sellPrice * (10**18))

You have a few options here, you can either set sellPrice to not be scaled up to units of wei 100, not 100 * (10**18). Or if you prefer to keep everything in units of wei, you can simply divide your final result by 10**18. Alternatively, you can keep sellPrice in units of wei, and what you pass in for amount don't convert to units of u