Non-scam: Someone you know and trust takes you aside and says Psst! You could ask a lot more for that domain.
Scam: All other scenarios leading to the same utterance.
Now, if you were feeling unkind, you could change all contact info on the domain,* and then send a reply e-mail saying "Thanks for the tip! I've just sold it to someone else for 40x my original price" ... and wait for incoming e-mail at the new address. But don't hold your breath.
* Replace this clause with whatever is needed to make it work in real life.