In my experience the condition of the store is a good indicator of the condition of the livestock. If they don't care about how the store looks, smells or is kept then they certainly don't care about how the animals look, smell or are kept.

If you have another outlet to get mice I would go there instead. The condition of how the mice are kept is one thing...but you also want to look to see that the mouse itself isn't sick (looking for a drainage from the eyes, nose or ears), runny stool is a bad sign, mice stink so unless you know what to smell for this is a hard one to guage.

Just remember in the wild a snakes dinner will come with all sorts of junk on the rat so the snake system can handle it...but avoiding as much as possible will help to cut down and anything it system can't handle.

Good Luck