When you make a purchase in the Marketplace, you will get instantly redirected to a page with the ISBN.
From there, you click over to Amazon and make your purchase.
You can see all your purchases on your My Purchases page in the Marketplace.
This can happen. (There's a weird bug with PayPal where about 1/300 transactions on the Marketplace don't register). If this happens, here's what to do:
-Peter