As buyers, should we expect dealers at cons to beat or at least match ebay auction house buy it nows?
No, I never expect it as a buyer and I give a slight premium to being able to look at every page of the actual book. Ebay can be really dicey for certain books; I've had sellers try to send books other than pictured among other things, that can be a pain to deal with.
I'm fine negotiating, but I don't like pointing out I can buy the same graded book cheaper online.
Just ask what their best price on x is? If it's close you know you're in the ballpark, if you're worlds apart just move on.