The Discogs site has been a good place to track a vinyl collection and buy/sell online to interested collectors. Apparently they now are set up so that CDs and tapes can be listed too.

This is not a quick solution for people who need to dispose of stuff right away. But for those who can be patient, it could be an option, especially for desirable rarities, or items you have multiple copies of. Take the time to make one listing, set the quantity to n, and see what happens.