I have three cassettes in good condition that are just lying around my apartment taking up (a very minimal) space.
Going to list them on eBay, but... I most likely won't have any luck with them. I suppose it's not so strange that they are inexpensive; the technology is virtually dead. (I will say that I keep receiving recorded cassettes from various language instructors, although one of my last did start putting .mp3 files on discs for us.) There also (probably) isn't a rabid fan-base for these recordings as there exists for vinyl (although I hear that has dropped off greatly as well).
Anyway, this is the first of many attempts to eBay my crap out of my apartment. I suppose these cassettes are a loss.