Base code for meetup

ERC20.sol

pragma solidity >=0.5.0 <0.6.0;
import "https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/contracts/math/SafeMath.sol";

contract ERC20 {
    using SafeMath for uint256;
    
    uint256 private _totalSupply;
    mapping(address => uint256) private _balances;
    
    event Transfer(address indexed from, address indexed to, uint256 amount);
    
    constructor(uint256 amount) public {
        _totalSupply = amount;
        _balances[msg.sender] = amount;
    }
    
    function transfer(address to, uint256 amount) external returns (bool) {
        require(_balances[msg.sender] >= amount);
        _balances[msg.sender] = _balances[msg.sender].sub(amount); // Remove the amount from msg.sender balance
        _balances[to] = _balances[to].add(amount);// Add the amount to "to"
        emit Transfer(msg.sender, to, amount);
        return true;
    }
        
    function totalSupply() external view returns (uint256) {
        return _totalSupply;
    }

    function balanceOf(address who) external view returns (uint256) {
        return _balances[who];
    }

}

marketplace.sol

pragma solidity >=0.5.0 <0.6.0;

import './ERC20.sol';
import 'https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/contracts/math/SafeMath.sol';
// 1000000000000000000
contract Marketplace {
    
    using SafeMath for uint256;
    
    ERC20 private token;
    uint256 private price;
    address payable private wallet;
    
    constructor(ERC20 _token, uint256 _price, address payable _wallet) public {
        require(_price > 0);
        require(address(_token) != address(0));
        require(_wallet != address(0));
        token = _token;
        price = _price;
        wallet = _wallet;
    }
    
    function buyToken() payable public {
        require(msg.value > 0);
        require(token.balanceOf(address(this)) > 0);
        require(msg.value.mod(price) == 0);
        uint256 tokenAmount = msg.value.div(price);
        require(token.balanceOf(address(this)) >= tokenAmount, 'You do not have enough token bro');
        token.transfer(msg.sender, tokenAmount);
        wallet.transfer(msg.value);
    }
}