I would think that if you're meeting locally, you should just check over the machine thoroughly and bring some USB peripherals to test. If it looks good, and it "feels" right, then you're probably good to go.
Having said that, I've had a few ripoff experiences myself. The first involved not testing, the second was not listening to my gut instinct.
The first was a laser printer that I bought from a guy and we met up in the parking lot of my workplace. Stupidly, I did not turn it on or try it out before handing over the cash. I just trusted the seller. It printed a few pages and then died. The seller stopped responding to my emails at that point, and when I called him out on it in public (Usenet forum), I began to get threatening emails from his buddies.
The second was a G4 Cube. I met the seller at his house where he gave me a cursory demo of the machine. At one point I bumped a cable and the whole thing powered off. He shrugged and said "power cable must have been loose". Something inside me said "this isn't right" but I was so excited by the prospect of a beautiful G4 Cube that I bought it anyway. Now I have a G4 Cube in my basement that powers itself off at random for unknown reasons.