Buyer wanting money back on motorcycle sale

[AVRacer]

I recently sold a motorcycle for $6000 and I signed the title to him and after he left I took the cashiers check to my bank and deposited it. Before he left I gave him pointers on were everything was clutch, brakes, ignition switch, owners manual, etc. He left happy. Two days later I found another motorcycle to buy and bought it with the money he gave me. I then got a call the next day from my buyer and he told me the bike would die when he stopped at stop signs and lights. I told him I didn't know what was going on and that I would look at it and test ride to see if it would do it for me. That never happened. He called one evening while I was at dinner with my family and stated that he wanted his money back or he would take me to court. I told him the money was already spent and that I couldn't get it but I would look at the bike and see hat it was doing. Is there anything that I should be worried about? I don't like selling crap so I don't. I owned this bike for years and never had a problem except for normal wear and tear (oil changes, batteries, tires,etc.). My co-worker ride the bike about a month before the sale and my brother rode the bike for about a week and a half neither of which reported problems to me. I rode the bike before I sold it and there was no problem. Will this go to small claims court or will could it go further than that? I live in Oklahoma and haven't had time to find any iformation as I got the call last night while at dinner. I will answer all questions and accept all information greatfully. Thank you.