I would bet that had you pursued the transaction something like this would have happened. The whole $5000 thing is just to see how easy you are to manipulate.
Yah, I used Edmunds and the Kelly Blue Book value for a car in “Good” condition. According to their criteria my car is somewhat less than good as it definitely needed a new set of tires, plus had cosmetic issues (scraped rims, dented trunk hood, peeling window tint, a patched tear in a leather seat); I hoped the 7-year-old custom audio system (still sounds great but none of the components are still in production) would offset it.
Apparently I was right, it sold to the first person who came to see the car at asking price. In fact that would suggest I set the price too low. But I’m fine with getting what I got for it.
I guess I’ll never find out what the deal was with those two guys offering $5k even sight unseen. In a way that’s better, I get to invent lots of back stories with no evidence necessary. 
I don’t think it sounds like much of a scam but more of just how these guys haggle. Seeing if you will accept $5K tells them how willing you are to negotiate. From that point even though they make a verbal agreement for $5K it’s still not guaranteed. They can now come over knowing you’re holding the car for them and start to pick it apart telling you why it’s actually worth less now that they are looking at it and all they’ll give you for it is $4400.