pragma solidity ^0.5.6;

contract Contract {

    struct transaction{
        address buyerAddr;
        address sellerAddr;
        uint priceOfItem;
    }

    mapping (address => uint) userBalances;

    mapping (address => transaction) transactions;

    function settlePayment(address transactionAddr) public {
        address buyerAddr = transactions[transactionAddr].buyerAddr;
        address sellerAddr = transactions[transactionAddr].sellerAddr;
        uint itemPrice = transactions[transactionAddr].priceOfItem;

        userBalances[buyerAddr] = userBalances[buyerAddr] - itemPrice;
        userBalances[sellerAddr] = userBalances[sellerAddr] + itemPrice;
    }

    function buyItem(address buyerAddr, address sellerAddr, uint itemPrice) public {
        address transactionAddr = msg.sender;

        transactions[transactionAddr] =
        transaction(
        {
            buyerAddr : buyerAddr,
            sellerAddr : sellerAddr,
            priceOfItem : itemPrice
        });
    }

    function deposit(uint amount) public {
        address user = msg.sender;
        userBalances[user] += amount;
    }

    function register() public {
        address user = msg.sender;
        userBalances[user] = 500;
    }

    function unregister() public {
        address user = msg.sender;
        delete userBalances[user];
    }
}