Ditto.
If it was just someone inquiring about purchasing the car, I wouldn't necessarily be weirded out. However, that bit is definitely an anomaly and changes things.
Not only that anomaly, but the fact your mom had weird feelings. Intuition should be recognized and not ignored, and combined with something that is not normal behavior by any objective standard, I think it's reasonable to suspect that your parents were being targeted for something.
A lot of times going to the police won't yield anything. However, there's plenty of cases that have been solved by dumb-luck of random tips from people who saw something weird and reported it....that whole "if you see something, say something". It could be worth it just to shoot it over to the local PD, as it's entirely possible they're tracking someone targeting old folks. If so, there's now likely video surveillance of his face/his car. Plenty of criminals get caught when they get overconfident and make mistakes, like being bold and contacting people in public areas with video surveillance.
If the PD doesn't respond with anything of interest (either legitimately or because the guy who answers the phone is just fucking lazy), then no-harm no-foul.