If it was just one sickly looking snake among otherwise healthy looking animals, there probably isn't much you can do. Every pet shop will have some sickly animals from time to time. It's just poor business practice to put those out where customers can see them. They usually stay in the back until they are healthy enough to sell. I'm not suggesting that most pet stores actually go to much time and effort to rehab their sick animals...just that they don't always put them on display. So one sickly animal among all the healthy ones isn't necessarily a sign of a shop that needs to be reported to the SPCA or RSPCA.

I also wouldn't consider buying such a snake. Buying it only puts profit in the pet-stores coffers and rewards them for keeping/selling sick animals. If customers continue buying the sickly ones (even with good intentions of "rescuing" the animal) then the store thinks it's OK to continue selling them.

What I WOULD do...is offer to take the sickly animal off their hands at no cost. (THAT is a true rescue) If they don't take you up on that (and most won't)...then make some suggestions about moving the snake into the back room where it can get some peace and quiet...as well as offer some ideas for rehabbing it. Again, it's unlikely that they'll take your suggestions, but it never hurts to try.