How about that dealer who sells you a comic early in the day for $10 then sees you walking by his table later on and calls you over to inform you that he just found out it was worth $30 so now you owe him $20?
Seriously though, I have never displayed my own comics at a convention but I have worked for a few people who have and I know both types.
One of the best is Mark Overby of Black Dog Collectibles based here in Richmond. He runs a tight ship. All of his books are bagged, boarded, priced and organized sensibly. It's mostly alphabetical and grouped where books are near other related titles. Such as if he's got a bunch of Batman comics together he will add Harley Quinn, Joker, Robin, anything Gotham at the end. If you are looking for something and he's got it then he will know exactly where it is.
He always tries to bring a good mix of high and low priced comics, he almost always has several key issues and he is very fair with his pricing. Mark delivers excellent customer service and he also has been known to track down something you might be looking for. He buys collections all of the time and gives you fair value if you sell to him. He has the most integrity of anyone I know in this business and I highly recommend him. If you are ever at an East Coast convention tell him that Howie said, "Hi!"
On the other end of the spectrum I have worked with someone who shall remain unnamed (initials are BC) who's ship is not as watertight
as you might wish. Inside Joke for those of you who get it.
This person has two approaches. The first is to sell minor "keys" for grossly overinflated prices and the second is to bulk out overgraded, robograded dreck for more than it's worth. Comics are in haphazard order, doesn't know going market prices or blows them up because of speculation. You won't get any kind of deal from him and you usually come out feeling a bit unclean afterwards.
He is definitely the worst of your Two Types:
a) That large stock is more likely to have a lot of comics that are complete garbage.
b) More likely to have their stock unsorted.
The worst kind of these dealers: Will see the cheap comics you've bought and try to pawn off their overpriced key issues that fit with them.
b) More likely to raise prices on comics related to writers and artists at the convention.
The worst kind of these dealers: Has people looking through other dealers and will buy and mark up and try to flip issues at the same convention.