To protect both buyer and seller, exchanges are handled by escow. This enables the seller to have bitcoins in secure escrow until the buyer pays. After the seller receives payment, Bitcoins are realeased to the buyer instantly.
The seller's bitcoins are safe and tightly secured.
The seller can not revoke Bitcoins after the buyer has joined a trade.
The trade can not be cancelled by the seller, only the buyer can cancel an on-going trade.
Escrowfix is nicely developed to display correctly across different device types, screen resolutions and changing orientations.