When trying to buy a domain name from a person or company in private, I frequently come across private registrations. I usually use the Whois history tool to see if there was an email address previously on file. I concurrently visit the website to see if there is contact information on the page.
Oftentimes people or companies acquired a generic domain name many years ago, and sometimes their projects failed or the domain name isn’t being used any longer. When the Whois history tool doesn’t help and there isn’t any current contact information on the website (or the site doesn’t resolve), there is one great option to find contact information: Archive.org.
Simply visit Archive.org and search for previous iterations of the website. You may find a phone number, email address, or the name of people previously involved with the website and domain name. This information can be used to inquire about a domain name.
Think about it this way. Most of the low hanging fruit has already been plucked. The great generic domain names that may still be acquired are out there, but many require sleuthing and research. Archive.org is another great tool to use to contact domain owners.