I wouldn't suggest limiting yourself to specific wheel sizes when buying on craigslist, cuts out too many good options.
IMHO full suspension is the difference between fun and kinda-fun. And I'm not sure why people think they need lock-out, that's just for a cheap, poorly tuned, or poorly designed rear suspension. They seem to have stopped even putting it on full suspension bikes the last few years.
In general, try to find a bike on craiglist that would be expensive new, and don't get anything downhill-specific, obviously. The most difficult thing about a mountain bike is figuring out what matters and what doesn't, so here's my recommendations:
hydraulic disc brakes - doesn't matter, but high end bikes usually come with it
disc brakes in general (vs V-brakes) - hugely important to me - V-brakes basically stop working when your tire rim gets wet.
8/9/10 speed - doesn't matter much
steel vs aluminum - steel weighs a ton, definitely aluminum
full suspension vs hard tail - definitely full suspension
derailleur (LX, Deore, Ultegra, etc) - only matters if you're hardcore.
When buying a used mountain bike, it is not uncommon that the chain has never been replaced. This often means that the whole drive train will need to be replaced (~$100 and probably 1 to 2 hours of work, not to mention several special tools), because a stretched chain will destroy the crankset and cassette. To tell if this is the case, try pedaling under load (i.e. up a drive way that is on a hill) in a few different gears and see if the bike automatically shifts gears under this load. If it does the drivetrain will probably need replaced eventually. (there is also a chain stretch measurement tool, but let's not get carried away...)
Given that potential difficulty, the ideal craiglist listing includes a line like "I bought it but only used it twice LOL". That's craiglist gold right there if you can find it.