Okay, but you already know that the cheapest thing about racing is buying the car ;)
here are some general guidelines:
1. Chassis should be year 2003 or newer, as anything older than that gets much harder to find parts for. Axle size can be 40 or 50mm.
2. If you get a shifter, a honda is the preferred motor just because of parts availability for karting applications.
3. If you get a single speed, it should be a Rotax or Leopard (IAME/Parrilla) for parts/service availability
4 In my experience, the hardest thing to find parts for on an older kart is the brakes, as there a dozens of different caliper designs and they all require a different pad size/shape. You might want to look into that before you buy.
5. Most of the stuff on your local craigslist is worn out crap that will not meet items 1-3, but if you look hard and long enough you’ll probably find something.