Withdrawing ethereum from a smart contract (return to sender)

Question

Let's say I have a decentralized application that allows people to buy some object at an arbitrary price.

User 1 purchases the object for 1 eth. A few moments later, user 2 purchases the same item. How can I monitor for this event and automatically return the appropriate amount of etherium to user 2? Here's what I have so far:

contract Adoption {
    address[16] public adopters;

    function returnEth() public payable {
    msg.sender.transfer(msg.value);
    }
    // Adopting a pet
    function adopt(uint petId) public payable returns (uint) {
        require(petId >= 0 && petId <= 15);
        //require(msg.value == 1 ether);
        adopters[petId] = msg.sender;
        returnEth();
        return petId;
        //return value;
    }
    // Retrieving the adopters
    function getAdopters() public view returns (address[16]) {
        return adopters;
    }


}

Answer 1

Reading between the lines, I think you're saying that user 2 shouldn't be allowed to purchase the item that user 1 already purchased.

If that's the case, then the call user 2 made to purchase the item should fail. You haven't shared the function that is used to make a purchase, but it should revert the transaction if the item is already sold.