Do you take old textbooks or just paperbacks? What’s the average price you give for a paperback?

Call them

we do not buy textbooks - try Walls of Books in DC. For paperbacks, we pay 10% of the original cover price. BUT we only give trade credit for SciFi, pop fiction and mysteries.

I’ve not seen them take textbooks.

They only pay 10% of cover price for paperbacks and 20% for hardbacks, even in new condition, and sell them for about half the cover price. And they're very selective and limited about what they'll take. You'll get double if you take store credit instead of cash.