I don't look at $1 books. You're right. But $2 books, I just take my pile off to the corner and quickly zip through them. I don't want to spend $2 on a book that will become a freebie. I usually buy 100-200 books like this at a show, if 5% are wastes, it adds up. In exchange for me taking up space, I don't try to haggle other than a minor round down...if I have $212 worth of books, make it $200? Most dealers do that without me asking. If I can't examine them I'd want a much better round down.
And, honestly, no dealer has told me not to inspect the books (even the guy who sells the ones with the bad back covers...indeed I regularly spend money with him at shows, not a lot, sure, but I look at the books closely). I would not expect a dealer to open $2 or $3 books. I may even say "if I cause a tape pull, that's my problem." And usually I have a big stack, so they probably don't want me to walk away.
Obviously it depends on how busy it is, but it is pretty rare that I can't find a corner to do this. And when I make my stack it is my intent to buy everything in the stack unless I see something I really don't like upon examination. Also, I AM NOT looking up cheap books on ebay, so that speeds up the process. Frankly, what I do takes a lot less time than some guy going through a long box and looking up every third book.
With that said, in recent years I have been limited to NYCC. I used to go to 3 or 4 shows a year. So most of the guys I am buying a bunch of stuff from recognize me (Zapp, JS Comics, Dolgoff (who may no longer come?), Alex MVP, and a few others) and know I am not there to pull out 6 million comics, take up a lot of space, produce a lot of odor, and not buy anything.