I understood it as, money goes to Amazon but then it immediately moves to the merchant. Showing the money as a revenue then eventually an expense. They probably hold that revenue for a certain financial period and write the expense in another. IANAA.

I looked into their annual report:

http://phx.corporate-ir.net/phoenix.zhtml?c=97664&p=irol...

In 2010, they say "Generally, we recognize gross revenue from items we sell from our inventory and recognize our net share of revenue of items sold by other sellers."

(Page 19, the second paragraph)

portman's comment could still apply to sales where Amazon acts as the merchant instead of just the storefront.