I’ve bought most of mine on craiglist but either way you sort of have to know what you’re doing. I’d say just sort by price and scroll through the results in the range you want to pay. You might also filter on “within 25 miles” or whatever, and get a unit that you can pick up in person. Or at least, try to talk to the person on the phone to get a sense that they know what it is that they are selling.

Thinkpads forum is still around and has a forsale section. It has slowed down but is still active:

forum.thinkpads.com/viewforum.php?f=11

I’ve bought stuff from there in the distant past.