I used to manage a LFS. It was ridiculously awesome. It was truly designed for the hobbyist. We had over 200 FW tanks and over 75 SW tanks. Plus 16 display tanks that totaled over 3500 gallons. The whole store itself was just a little over 10,000 square feet. Anyway, we had about 15 to 20 different vendors we could order livestock from. Some were good, some bad. Generally we narrowed it down to the good ones and ordered from a select 4 or 5. I would say about 100 to 200 fish total would be dead between the day of and day after receiving fish (when you factor in that we ordered Danios in by the 100’s, its really not that much). But that was, say… 3 guppies, 5 danios, 4 white clouds, 6 neon tetras, 1 pleco, 4 blue damsels, a royal gramma... Etc, etc. You have to figure with that many tanks; we are BOUND to miss some.
Now with a small store, its harder to miss a fish if some is looking around real well. Find out when the store gets its shipments. If it is the day of or day after, expect some dead. If not, look at how dead the fish are. If they look like they are fresh, (just died that day, still have decent color, not really rotten, still has form) then oh well, not that bad. If the bodies are rotting, snails all over it, already half eaten, then I would pass. Another thing to look at is how many in each tank. Say you go in, and a bunch of neon tetras are floating at the top. But that’s it; nothing else in the whole store is dead. And again, they look “fresh,†then I wouldn’t worry too much. Those fish may just be crashing. But if you go in and there are 3 dead fish here, 2 there, 5 over there, and a whole tank full somewhere…. well, you know what to do.