I can't comment on the merits of the lawsuit, but if you are deemed to have breached a contract you can certainly be sued in the state where the contract was breached, even if you have since moved.
The information provided herein is general information only and not legal advice. The information provided herein does not create an attorney client relationship and is not a substitute for having a consultation with an attorney. It is important to have a consultation with an attorney as the information provided in this forum is limited and cannot possibly cover all potential issues in a given situation.
Based on the information provided, I believe that the statute of limitations for New Jersey is six years on a contract dispute. So yes, the buyer can sue you in New Jersey. Please be advised that I am not licensed in New Jersey, and therefore you may want to contact a New Jersey licensed attorney for additional information and advice.