It's definitely a scam - I received the same email, answered I'm selling the domain for $5000 - he asked for appraisal and of course, suggested a "professional appraisal site" as well - google didn't really give any results for it. And when I said that if he's interested in the site then he can get the appraisal himself and he can later pay the asking price minus the money he paid for the appraisal - well, I never ever get an answer from him anymore lol.

Cheers
Siim