Been to loads, They are not common these days, but they are a good place to pick up new stock, as many breeders sell on batches (usually six per bag) of young fish which they have bred.
Or you could even find unusual or surplus breeding stock. A lot of fish auctions are run by aquatic societies, so they are generally a responsible lot - there should be no deseased fish or dodgy equipment. As a rule all electrical equipment should be in working order, sellers are usually asked to place a contact address on electricals as a gesture of good will should anything be faulty.
You are usually allowed to view lots beforehand, make notes of which lot You are interested in & good luck