i'd do two things:

find a trustworthy employee, one that is sympathetic and knowledgable, and try to find out where they get their bp's from. if it's cb then ask when the last time it ate was, also if they've had mite problems, and a bonus would be taking it to a reliable vet. i know you're strapped for cash but waiting to save up a bit and buying from a breeder would be choice. i totally agree with lily on the fact that most sell wc. if that's the case- pass it up. i'd also try and convince your parents because any snake is a commitment and if you "want to get the little guy out of there" then you should make sure he has ideal conditions at your home, ie a separate tank. your parents may say no and that just means you gotta wait, i know that's life and there is no way if i hadn't moved out that my parents would let me keep another. if all that doesn't work (finding out wc or cb) and convincing your parents to let you keep another tank then you might as well try and do the right thing and let the store know about the conditions they are keeping their bps in, try to get them to set up another tank or get them to do regular cleanings and make sure the set up is correct- multiple hides, the temps and all that. hope that helps and good luck!

-billy