The ones I really hate are the guys who try to sell you something else. "Here, you'll like this one a whole lot better!" Not likely, since it's totally different from what I'm looking for. What he really meant was "I don't care what you want, just buy this one."
You always have to watch out but if the show isn't too crowded you can usually take a few minutes to talk to them, get to know what they're up to. Along with all the other characters I've met some pretty good folks at some of those tables.