We're testing an idea, not announcing a final policy. Please judge these terms as they are.
Here's one way it might work:
Bring in books in good condition. We review them and select the ones that fit our shelves. For accepted books, we offer store credit, not cash.
Example: A book we'd price at $10 might earn you $3 in store credit.
Under this possible plan, credit could pay for up to half of each used-book purchase. If you bought $20 in used books, you could use $10 in credit and pay $10 yourself. Unused credit would remain available for later visits.
This is just a proposal, and not every book would be accepted.