Today when I went to www.amazon.co.uk it immediately knew who I was (without having to log in) and what I've bought recently. It then calculated how much I could make if I sold these items using the Marketplace
This a brilliant use of web technology since it's not an emulation of what can be done in a physical store. I.e. you would never be welcomed by a sales clerk in a shop who says: "Hello again Peter. Last month you bought X, Y and Z. If you want to we can attempt to sell it on to someone else and we take only a small share".
I don't know what Amazon's cut is on the profit but obviously they've taken all factors into play. They take a cut on selling a DVD, then they can take a cut on selling it again!. Plus they get all the other marketing benefits with it at the same time.
As soon as I'm bored with the DVDs I bought that I can sell I will attempt sell them using this feature.