I was a happy member of the bookateria for several years while attending the University of Delaware as a student, and over this period dropped off many of my science fiction/fantasy paperbacks with them for credit. Limited dorm room space necessitated maintaining a small bookshelf. I accumulated approximately $390 in credit, with roughly 30 scifi credits (I traded in hundreds of SF/F books, to purchase a scifi book with your $ creds you need scifi creds on a one-one basis).
Eventually I graduated, and left Newark for another part of the state where I had found employment. About 3 years later I stopped by the Bookateria, looking forward to redeeming more of my (presumably still valid) credits, only to be told that my card was no longer on file and that the hundreds of dollars worth of credit I had with them would not be honored. It wasn't that they had lost it... They unabashedly informed me that they had purged their files as a matter of routine, assuming that I (and presumably other accounts) had left Newark never to return.
There is no policy posted in the store about credits "expiring" after a certain period. I left hundreds of dollars worth of paperbacks with these people in good faith, fully expecting that my credits would be valid in perpetuity. My recommendation would be to avoid this store at all costs, but if you simply must trade with them, only accumulate as much credit as you need each visit, because if you don't return right away it's likely to vanish!