For specific music Discogs.org is a very good marketplace. It's not free, it's a second hand marketplace, people don't give away things, but prices aren't worse than retail except for actually rare collector's stuff. But I've used it a lot to fill in the gaps in some artists' discography. Also older CDs are usually mastered with a bigger dynamic range, so they sound better on music hardware with a functioning volume knob. ;) So I much prefer buying original CD releases versus re-releases of an album on CDs anyway.