Knowing the receiving address is trivial since I would display the associated QR Code on my website.
I guess the issue is to know the transaction id when a sell is made, so I can validate it!
If you know the receiving address the seller is using, and assuming that they are using new addresses for each payment, you can watch the Bitcoin network for any transactions that send to that address. If the seller is using unique addresses, then once you see a transaction sending Bitcoin to that address, you will know that the seller has paid. This is what the seller's Bitcoin wallet is doing in order to show the seller his transactions and Bitcoin.