I practice criminal law. This came up in the criminal fraud section. You did nothing criminal. My guess is the sale was "as is" especially if you gave him the opportunity to check the car. But I do not know "lemon law". I do not think it applies to private sales. But I would check with an attorney who practices commercial law or deals with "lemon law".
Some states have lemon laws on used car sales. Some states have lemon laws on private party sales. I am unsure if NJ is one of those states. Regardless, this person had the vehicle checked out, and drove it for two weeks, so, it's highly unlikely they can pin something on you, UNLESS you knew there was a problem and kind of rigged the thing to run for a while, which may be a fraud, but, even that would require the mechanic not noticing it.
Of course you've heard that anyone can sue anyone for any reason, so, make sure you have all of your documents together and you are ready to hire counsel, should you need someone to represent you.
If you are not a professional seller you are subject to suit for breach of warranty and many other possible claims (breach of warranty).
If you are a private (three or less cars per year) seller than you need to be concerned about the Consumer Fraud Act, which applies to private and professional sellers alike. He would be within his rights to sue you for misrepresenting the condition even if you did not know about it.
Under New Jersey Law affirmative representations are actionable (you can be sued) if you make them and they are false, and no knowledge or intent is required. So if you said the engine worked and when he received the vehicle it did not work that could be deemed an affirmative representation and he could sue you for triple damages and attorney fees
The short answer is call Jon Rudnick; he knows your state laws and follow his advice. Now, here's the long answer for you: In a private sale, in most states the seller’s only obligation is to answer questions truthfully, not hide anything, tell the truth about the vehicle mileage, and sign over a clear title to the vehicle. Still, buying or selling a car in a private sale can be risky business because the law is very different from a car dealer sale. In a sale between two people, neither of whom is a car dealer, in most states the only obligation on the seller is to answer the buyer’s questions honestly and not hide anything that the seller realizes the buyer would want to know about. The seller has to tell the truth about the mileage on the car too by filling out correctly and honestly an odometer statement for the buyer to have. And in those states that require mandatory emissions tests in order to get a vehicle licensed, many of those states say that if the emission/pollution equipment was disabled or removed then the buyer may have the right to cancel the sale. There is a used car lemon law in Hawaii, Massachusetts, Minnesota, New Jersey, New York, and Rhode Island, but nowhere else so far - and most of those only apply to car dealers and not private sellers. You can read an outline of your state's new motor vehicle Lemon Law on this web page here: http://ohiolemonlaw.com/state-lemon-law-summaries.htm. Next time, if you want to make the agreement clear so no dispute comes up later, the buyer and seller should consider writing out a “contract of sale” which says what the year, make and model and “VIN” number of the vehicle is, and also what (if anything) is being represented about the vehicle or if it is being sold “as is,” and then both of them sign it and each keeps a copy. But a written contract is not required in most states to privately sell a motor vehicle. Car dealers, however, have to use a written contract in every sale. To find out for sure what your obligations and rights are in a private vehicle sale in your state, you need to talk to a local attorney who deals with this kind of case. Check this web page for a Free Online 50 State national List of Local Lemon Law Lawyers (they don't pay to be listed here and most of them are members of the only national association of consumer law lawyers): http://ohiolemonlaw.com/locate-a-local-attorney.shtml. You can also look on Avvo.com under the Find a Lawyer tab or call your local attorney bar association and ask for a referral to a lemon law lawyer near you. If this answer was helpful, please give a “Vote UP” review below. And please be sure to indicate the best answer to your question so we can all be sure we are being helpful. Thanks. Ron Burdge, www.BurdgeLaw.com
This answer is for general purposes only and does not establish an attorney-client relationship. The law in your state may differ and your best answer will always come from a local attorney that you meet with privately. If you need a Consumer Law attorney, click the link above to find a Consumer Law attorney near you.