Likely craigslist unless you're willing to make some local connections.

Be aware this equipment is not pleasant to be around (loud, hot)

Server hardware is too heavy/bulky to be able to support a big online marketplace. Your best bet is to identify the type of organizations that buy the server hardware you want to buy used and find out how they get rid of their excess.

I've gotten a handful of free server equipment (no drives) by being willing to show up and take whatever is being offered.