I'm reading your question as being interested in a car advertised in the "marketplace" section of this website.
My one suggestion is to remember that you are looking at a used car, private sale. I've had good experiences buying cars on ebay, but always make sure you know who you are dealing with and thoroughly research, inspect, etc.
For buying a used tesla, don't assume the car has all the features expected or advertised. Tesla regularly changes the features included in each model, not just once a year like traditional auto manufacturers, so model year off the VIN does not easily tell you what is included. So verify AP is as advertised by looking at menus on display, etc.