A broker calculates profits based on the "last trading" price?

[johnymm]

Is this possible?
All P/L should be calculated based on bid/ask, right?

For instance: If I buy at 80/ask and sell at 79/bid = -1$.
But now I am testing a demo software of a broker and they seem to calculate all profits/loses based on the "last trading" price. For instance: bid/ask = 80/81 - last trading price = 90. So if you buy at 81 and sell...you sell at 90??? I know it's wrong, i am not hoping even a 0.00000000000000001% fo this to be real and arbitrage way to make money. My question is more like: Why don't they change their trading platform?

10x

 

[Rhody Trader]

There's no arb here. What your broker shows as P&L is strictly an accounting rule. Your final P&L will only ever be determined by where you execute your trades. If you buy at 81, as in your example, the last traded price (assuming no others trade immediately after you) will be 81.