Red tail catfish, arowanas, giant gouramies, and red bellied pacus should not be sold to hobbyists period. You need a multistory municipal aquarium to house these monsters!
Oscars, bala sharks, common goldfish, clown loaches, and common plecos should be sold with a warning that they will eventually require a relatively large tank. I'm sure there are several other fish that I didn't mention that belong in this category.
$70 for adult albino dwarf pleco?!?!?! These guys breed like crazy. I can get 1 inch juvies for about $2 each at my aquarium society's monthly auction.