he just wants the $400 he spent to fix the frame from me

This is exactly the point. He wants $400 because he says he paid that to get the car fixed. A: You have no proof of that and even if you did have proof you have no reason to pay him. You sold the car "as is" I bet you didn't even write up a proof of sale. I doubt he demanded a receipt either. The long story short is that he had time to inspect it. He had plenty of time to find any issues (30 days by law you say) and he chose not to. He chose to decide 2 months later that "oh this isn't exactly what I wanted" Whelp. This is why you do your research before purchase. Tell him to pound sand.

IF he sues you, he will lose unless he can show PROOF (usually in writing) that you misrepresented the vehicle or committed fraud. Neither of which seems at all likely given your facts so far.

Another thing about small claims court, Its not free. He'll have to pay to file the case ($50 here, dunno where you are) and he'll have to pay to serve you ($11 by mail here) or if you refuse service typically around $50 to have a process server do it. So basically, around $111 to even start to dispute something for $400 in which his case is almost non-existant. So, his absolute best case scenario is to get 75-80% of "his money." At a significant loss of time and travel (you said he's in another state) parking, getting off work. ETC. He'd be a fool to actually follow through. As such, he's simply trying to scare you. I imagine his next step would be to send a nasty-gram (Demand letter) saying formally "If you don't agree to send me money, I will sue you." If you ignore that, he'll either let it go or he'll be really stupid. Either way, you've done nothing wrong. It's all part of sales. Sometimes people are pissed about what they bought and you are the only person to take it out on. It's shitty but shrug that's how life works sometimes. Best of luck to you Cheers