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.

Check with your local non-profits, most 2nd hand no warranty/guarantee shops are in your local small industrial parks and sell mostly to non-profits or very small companies.

They will not have a store front or advertise you'll just call the owner and ask if they have "X" or equivalent and if so you can go get it. I've used a few of them over the years for monitors and wyse terminals for small businesses.

My advice is to search for the stuff you want on eBay, then filter results for sellers near you, identify some interesting sellers and try to visit their facility in person. If you buy through eBay, then a less awkward tactic is to choose "Local Pickup" instead of a delivery service. When you're at the facility, ask if you can take a look around or if they know of other sellers for used server equipment.

I have made a trade with someone local, worked well for me: https://www.reddit.com/r/homelabsales/

ymmv!

eBay routinely has listings for desktops with 8th to 10th gen intel CPUs and quite a bit of beefy hardware for under $200 overall. Some of them also come with GPUs in them.