There's absolutely no reason for any pet store to have a snake (or any animal) in THAT bad of shape. I'll gladly be called a rude ass for sticking up for the care of ANY animal and you bet your ass I'd make a fuss if I came across something like that. No hesitations.
Maybe it did come in that way. Totally possible. But you either A) take it to some sort of rescue society knowing it could get proper MEDICAL care whether it be tube feeds, IV fluids, antibiotics, mite treatments or B) if you're confident enough nurse it back to health in the store, do your company name a favour & DON'T put it on sale display.
If they've tried & it isn't responding to any feeding attempts, I second my previous comment. You don't let an animal get like that. Take it somewhere where someone can give it far more attention.