Why don't you go back to the seller and ask him calibrate it for you? That should prove right then and there if it is defective. Wouldn't hurt to have a witness along for the ride, who might happen to discretely be recording the episode. If at that point it doesn't work, ask for your money back. Might be that easy. If not, you would have evidence and a witness for your venture into Judge Judy's court.
P.S. A screenshot of the Craigslist ad would be most helpful as well.