Likely Facebook Marketplace or maybe Craigslist. eBay is pretty rough these days by all accounts.

eBay, yard sales, Goodwill, local thrift stores, Facebook Marketplace, etc.

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.

Also: Your relatives and friends. And their friends and friends' friends. Your neighbors. Ask around. And whenever someone moves, they usually want to get rid of things.