address owner = msg.sender;
IERC20Token token = IERC20Token(0x...);
uint256 rate = 10;  // 1 wei == 10 token units
function buyTokens() public payable {
    uint256 tokenAmount = msg.value * rate;
    require(token.transferFrom(owner, tokenAmount, msg.sender));
    require(owner.transfer(msg.value));
}