well first address them by their name eg. Dear Mr. Smith

next make it a formal letter. try something like this:

Dear Mr. Smith,

I came accross your domain name, http://www.asdf.com/, and would like to know if you were interested in selling it.

just be polite and don't use "u", "ur", or any other various internet lingo. also try not to send from a hotmail account or an unprofessional sounding e-mail address. so if you send one from sales@domain.com or domains@domain.com it is far more professional than w0ot_1_r_1337@hotmail.com. Use your real name too and not your nickname.

Hope that helps.