Other than telling them about the conditions, there's really nothing you can do to change it...

It's up to you, but you can't save all of them. If you get that one, you'll probably see another one in the same conditions in about a week. You can try to keep up with local reptile rescues. That way you know that the space you freed up by adopting one will go to another animal that really needs it.
Also, it's usually well-hidden, but most Petco/Petsmart have an adoption board for their sick or aggressive exotics that they can't sell. They're free, so the company doesn't profit from it. That's how I got my python (biting a couple petstore employees was probably the luckiest thing she's ever done). Be careful though; I've seen several frogs with severe and possibly contagious issues put up on those. Like someone already mentioned, be sure to quarantine whenever you bring in a new one.