all of the above,
appearance of the tanks is important if they look nasty chances are they are.
price if the prices are far above average there better be a good reason, such as all stock is captive bred. etc.
knowledge, if theyy cant tell me how to care for something properly they arent taking care of it properly while they have it.
selection is somewhat important, but not as important as healthy stock
live stock, if they dont have licve stock i dont walk in the door, there are millions of places to buy dry good.