Caveat emptor of course applies, but that can’t provide a defence for misrepresentation. So if the guy lies to you about the car, like saying he’s just had the clutch changed for example when he hasn’t, then you have legal recourse.
Desperately wanting cash only is perhaps a tad odd but it’s the only way to be sure really, aside from forged notes... But as above, you MUST get a signed, dated and timed receipt, preferably a copy each. If he won’t do that, then I wouldn’t buy the car tbh.