Some people like to put a thread in the gen discussion forum, advertising the fact that they are about to place an order on Amazon.com. When you place an order at amazon, you have a chance to enter emails for people you want to pass on a 10% discount to. Then, if these people in fact buy the product you gave them a discount for, you then earn credit you can spend on Amazon. Essetially, both parties save money.
I've done it twice in the last week, and I apologize if it's seemed like spamming to some people. The second time I did it for my order of Draconomicon; I put my promo in an already started Draconomicon thread instead of starting a new one.
Putting htem in the market place makes some sence; I guess like every other non-General Discussion forum if they're there they get less exposure, and people who make such posts want maximum exposure.