I would see what they mean by "great eater". To some, that could mean "one mouse every single month without missing a single meal ever"! I don't think it's necessarily a case of being on the lookout for a scammer, more of a being on the lookout for a less-than-knowledgeable keeper and being prepared for your new addition.
I'm also impresses that they'd be willing to let you take the snake to a vet pre-purchase. Good on them (and you) if that's the case. It's certainly not the norm.