Send an E-mail to the registrant and/or admin contact. It's important to note that registrant & admin contact are not always the same. Often the admin contact is a hosting/IT company and they should not be relied upon to forward your offer to the owner.

It's important to make sure your E-mail will not look like spam. Put the domain name in title and nothing else (the Sedo way).
Then write the name of the owner - if it looks personal they will be inclined to read on.

ATTN: Mr/Mrs [owner name from whois if available]

Dear Mr [domain owner],

I may be interested in your domain name xxxx.com.
Since the domain is not in use/appears to be parked I was wondering if it is available for sale.
Feel free to contact me
I would appreciate if you could let me know your asking price along with your reply.

Kind regards,
[potential buyer]


[leave phone & E-mail here]