Personally people should come to DP to discuss games primarily. The buying/selling part should be secondary imo. I know a few get annoyed with some users only here for the trade side of things, but there's nothing wrong with that so it's not like we can stop everyone from selling unless they've accrued XXX number of posts.
What about someone who spends all their time in the Off Topic forum talking about the NBA, movies or booze and doesn't ever make posts in the gaming forums? There are quite a few such users here. I could get on there and boost my post totals but any type of message board only holds my interest so long, gaming or otherwise.