What you pay for a book really has nothing to do with what it is worth later on.

Unless you are selling through Amazon or Abe, bookfinder prices are indicative at best. You will only see eBay listings on there if there are very few others. And you will also see a vast range of prices - because some sellers sell for peanuts, some have inflated ideas of what a book is worth, some are first edition hardcovers, Easton Press or Folio Society copies, or signed.

The important thing is what the book/s are selling for on eBay, the platform you are actually using.

Used Dan Brown paperbacks will not sell here for more than about $13, for example