safeRemotePurchase.sol

contract Purchase {
    uint public value;
    address public seller;
    address public buyer;
    enum State { Created, Locked, Inactive }
    State public state;

    function Purchase() {
        seller = msg.sender;
        value = msg.value / 2;
        if (2 * value != msg.value) throw;
    }

    modifier require(bool _condition) {
        if (!_condition) throw;
        _
    }

    modifier onlyBuyer() {
        if (msg.sender != buyer) throw;
        _
    }

    modifier onlySeller() {
        if (msg.sender != seller) throw;
        _
    }

    modifier inState(State _state) {
        if (state != _state) throw;
        _
    }

    event aborted();
    event purchaseConfirmed();
    event itemReceived();

    /// Abort the purchase and reclaim the ether.
    /// Can only be called by the seller before
    /// the contract is locked.
    function abort()
        onlySeller
        inState(State.Created)
    {
        aborted();
        state = State.Inactive;
        if (!seller.send(this.balance))
            throw;
    }

    /// Confirm the purchase as buyer.
    /// Transaction has to include `2 * value` ether.
    /// The ether will be locked until confirmReceived
    /// is called.
    function confirmPurchase()
        inState(State.Created)
        require(msg.value == 2 * value)
    {
        purchaseConfirmed();
        buyer = msg.sender;
        state = State.Locked;
    }

    /// Confirm that you (the buyer) received the item.
    /// This will release the locked ether.
    function confirmReceived()
        onlyBuyer
        inState(State.Locked)
    {
        itemReceived();
        // It is important to change the state first because
        // otherwise, the contracts called using `send` below
        // can call in again here.
        state = State.Inactive;
        // This actually allows both the buyer and the seller to
        // block the refund.
        if (!buyer.send(value) || !seller.send(this.balance))
            throw;
    }

    function() {
        throw;
    }
}