[Ethereum] Smart contract that accepts custom token

I think this should be your function:

function buyTokens(address _receiver) public payable { 
uint256 _amount = msg.value; 
require(_receiver != address(0)); require(_amount > 0); 
uint256 tokensToBuy = multiply(_amount, (10 * decimals)) / 1 ether tokenPrice;
require(tokenContract.transfer(_receiver, tokensToBuy)); 
tokensSold += _amount; 

emit Sell(msg.sender, tokensToBuy); }

Edit: You can check the amount of tokens received by the end-user via Metamask.

I suspect this is just a rounding problem. MetaMask doesn't show enough decimal places. The account that created the token has a balance of 0.000000000000001.

The token has been created with 18 decimal places, so it takes a balance of 10^18 token units to equal one token. If you want to have 1000 tokens, you need to pass in 1000 * 10^18 (a 1 with 21 zeroes after it).