If you can't get hold of the owners details very easily then that might be a good sign they won't be interested in selling.
Also remember that no website is in no way indicative of a domain not in use.
One of my "main" domain names is purely used for e-mail.
I also have another which only has FTP traffic associated with it.
If it is a .uk domain then individuals are allowed to opt out of Whois.
In which case it is like having an ex-directory phone number - only the company the domain was registered through and Nominet will know who the domain really belongs to and unless it's a legal reason they won't share this information.
If it's a .com, .net, .org or other TLD then providing false information in the Whois is actually a breach of contract.
Although many companies offer what they call "Whois" protection this actually goes against ICAAN rules.
However it is a grey area and unless you were trying to find the owners details for legal reasons (their domain is spamming, being used to illegally distribute files you own copyright on etc) nobody is likely to give you in the info.
In theory all domains should have a postmaster@ & hostmaster@ e-mail address which is live.
So you could try dropping an e-mail to either of those addresses at the domain name you're interested in - if they have a catchall for that domain that would also pick up mail sent to either of those two addresses.
Finally if you could find out what company the domain was registered through you could drop them an e-mail and ask them if they would be willing to pass on a legitimate offer to buy the domain and your e-mail address.
That would leave any further contact firmly in the court of the current owners.
