Sorry. But if a dealer just tosses unsorted cheap common books into 15 long boxes at a con, which forces someone to spend an hour digging through them looking for stuff, they have every damn right to decide they don't want stuff they pulled out. A) The dealer was an a-hole who made it a PITA to look for stuff. B) If someone takes the time and effort to look through a box of UNSORTED comics, they have the right to change their minds on any they pull out; 3) It's if they're pulled from the unsorted lot, it takes all of 5 seconds for the dealer to stick them back in a box!
If a dealer puts out boxes of sorted/alphabetized books, then its easier for the collector to find what he or she is looking for (or not), and move on. A LOT of dealers do not sort books for this reason, plus it's a lot of effort. They make more sales because they have a bunch of peole going through 10 long boxes looking for hidden gems, or to fill holes in their collections. So, do not complain about someone doing all of that, and deciding not to buy anything.