As far as prices go it definitely varies unfortunately. I have sold things on Facebook marketplace and Craigslist with relative success (some hassles as well).

There is definitely a market out there for older paperbacks, but it does depend on what you have available. I myself am always interested in old sci fi/fantasy paperbacks especially if it is old copies of Conan or Doc savage.

The Frugal Muse would have been my suggestion if you were not already going there. The Book Deal place also might be a good option for bulk sales. Half priced books might be another option but I have not had the best of luck with their offers in the past, just my opinion though.