Please do confront the people at the pet store about this. I bought a parrot at a shop once that died a month later from a congenital defect (meaning he was intentionally bred from sick parents) and I was too upset to go back there ever again, but I wish that I had.
My Duchess, it turned out, was also ill when I bought her, but nowhere near as bad as your baby. She had internal parasites and upper respiratory problems which we treated with injections, and she's better now but prone to flare-ups of URIs. It cost me a couple hundred the first time to get her well and she might have to go back. But I'll be straight with you, I think if she had all the problems your snake has, I'd probably put her down. That's a lot for an animal to have to live with; the cost is secondary but still an issue.