I can't agree enough with the comment about emailing someone from a real address.
I am currently negotiating with a pretty big law firm in a major city for one of my domain names. I can guarantee you that if they sent me an email from a gmail account I would not be in negotiations with them right now. Knowing their identity allows me to trust them and allows me to know they aren't just sniffing around.
In my previous example, the initial email came from a UK Yahoo address. Once we got serious and he suggested an escrow service, I checked him out there. They knew him as an active and able buyer.
Creating hard and fast criteria upfront can kill an opportunity prematurely.