BTW, respiratory infections can be contagious to other snakes nearby...& I'm betting this one wasn't in quarantine either?So the other snake could come
down with it after being stressed, coming into a new home (yours) if you go that route...only that might not show up right away, & then you would be stuck.
You didn't mention the gender of the adult snake but part of me is guessing it's an adult female, maybe this one's mom- that would explain the fact they're feeding
2 adult mice...but if now available for purchase, that means it's a retired breeder- no longer productive for them to make money off of, and breeders don't usually
live as long as un-bred snakes, as breeding takes a toll on their health. So without a lot more information, of the reliable (!) kind, I'd not chance it. My 2 cents.
Plenty of reliable breeders around with healthy snakes available.