A board game convention will be really cool to attend to see what's new and how it's played before I buy. Its really disppointing to buy a boxed game only to see that its not all it was supposed to be from what's shown outside the box.
I've only attended one board game convention, but that's not by my own choosing. I absolutely loved it.
If you go, be sure to bring several of your own games, an open mind, and a list of games you'd like to try before, as was said, you decide whether or not to buy them!
I'm sure it depends on the convention... It seems logical that a commercial seller would try to get in if possible.
At the one I went to, there was one stand in the corner by the entrance selling games, but I think that was being run by the convention's organizers.
That being said, there's nothing against selling or trading games with other participants.
My main problem with this kind of of setting is I tend to be picky with the people I play with. I feel bad when I have to turn someone down who is eager to play but I don't feel the same way about it. I would just be standing and looking on and that will be entertaining for the first fifteen minutes I guess and then I'm ready to leave.