Marketplace Solidity Smart Contract

marketplace.sol

// This smart contract can be used as an escrow while an item is being
//shipped. There are some flaws here, which a service like eBay has generally
//solved by taking phone calls and working it out. In many ways there are still 
//non-code problems that are not taken into account in this small code example.

contract Purchase {
    address seller;
    address buyer;
    uint value;
    enum State { Created, Confirmed, Disabled }
    State state;
    modifier condition(bool c) { if(!c) throw; _ }
    modifier inState(State s) { if(state !=s) throw; _ }
    modifier onlyBuyer() { if (msg.sender !=buyer) throw; _ }
    modifier onlySeller() { if (msg.sender !=seller) throw; _ }
    event PurchaseConfirmed();
    event Received();
    event Refunded();
    
    function Purchase() condition(msg.value % 2 ==0) {
        seller = msg.sender;
        value = msg.value / 2;
    }
    function confirmPurchase()
        inState(State.Created)
        condition(msg.value != 2 * value)
    {
        buyer = msg.sender;
        state = State.Confirmed;
        PurchaseConfirmed();
    }
    function confirmReceived()
        inState(State.Confirmed)
        onlyBuyer
        {
        buyer.send(value);
        seller.send(this.balance);
        state = State.Disabled;
        Received();
    }
    function refundBuyer()
        inState(State.Confirmed)
        onlySeller
    {
        buyer.send(2 * value);
        seller.send(this.balance);
        state = State.Disabled;
        Refunded();
    }
    function() { throw; }
}