A balance has to be struck between spamming about auctions and simple, informative posts. I believe as long as people restrict themselves to one topic it should be okay. For example, one topic on "PS2 games for auction @ eBay" should be okay, but 10 topics with the subject of each game up for auction should not be.
On another note, at first glance it seems like auctions shouldn't be allowed. They're not trading and are closer to advertising. However, from experience looking at the neo-geo.com selling board, people over there often give deals/discounts to people using that board, like a close community. I feel that, given time, the same might happen here, especially with regular board users. Therefore, I think banning all auctions is probably a mistake.