I agree with Daniel. As heartbreaking as it is to not buy this poor snake the fact is if you give this guy cash for a snake in that condition he'll figure he can sell any snake in any shape and make a quick buck. Not something you want to encourage. Like Daniel said, I would instead suggest to this store owner that this snake is in such poor condition that it's likely to expire and you as a vet tech have access to the means to save it if he will simply give it to you as a kindness to the snake. He obviously didn't put out any real cash on the snake and if he took it in trade he sure doesn't know what he's doing or he traded an equally sad animal for this snake. If you have a reptile rescue locally I would encourage you to call them as well for advice about this sort of thing.