I sold a car to a co-worker: Am I responsible for any refunds or repairs?

I sold a car to a co-worker, since he bought it the car has had one problem after another. Now he is talking about suing me. The deal was between private parties, there were no warranties discussed. I honestly had no idea these problems were about to occur. Am I responsible for any refunds or repairs?

You are not responsible as long as you did not lie or purposely hide some material problem etc. Your friend would lose in court.