Sold a car as is and now after a week, seller is demanding money back.

I sold a car on Craigslist and a week later they are having an issue with it and demanding money back. I did not knowingly sell the car with any issues. The car wasn't mine. I bought it from someone and sold it. The issue is when I bought it, the seller just signed the back of the title and then when I sold it we finished the paperwork with an actual notary and it went into their name. (It never was in my name) I was completely unaware that this was illegal. Am I legally obligated to give the money back? I thought if you bought a car from a private seller, it is assumed that the transaction is AS IS?