I agree with Larry and Joules. This doesn't add up. Besides, waiting a FULL YEAR to bring any issues to your attention is ludicrous! Sounds like he screwed up and wants you to pay for it.
Reminds me of a 1996 BMW 328 I sold years ago... The car had a new radiator, passed emissions testing with flying colors about 4 months before I sold the car - I replaced the catalytic converter a year or two before that - and it ran great!
Anyway, the buyer didn't want to wait an extra day for me to smog it again (since my last certificate was beyond 90 days old) so I discounted the price and he said he would take care of it. I sold the car "as is" and off he went.
A year later he called me to tell me the car had all kinds of problems and he couldn't register it, and wanted me to pony up $$$ to help fix the problems, including failing the smog test, leaking radiator, didn't idle right, and some other miscellaneous interior items he broke. It turns out he never took it for an emissions test or bothered to register it until my stickers expired. So he drove the wheels off of it (likely abused it) for a YEAR, then had the gall to call me and tell me that CA law states that the seller is responsible for the car passing emissions testing and that I needed to "be a gentleman and do the right thing" and, at minimum, pay for half of his $1,200 repair estimate!
I reminded him the car was perfect and had a new radiator when he bought it and had just passed smog 120 days before he purchased it, and that EVERYTHING worked and there were no issues. AND, that CA law ALSO states a buyer has 10 days to register a vehicle after purchase. I told him to "feel free to call the police, the CA Air Resources Board, the DMV, or try to sue me" if he felt so inclined.
I never heard from him again.