It’s probably silly for me to give away insight related to how I find contact information when Whois lookups fail, but I guess that’s one reason why my blog is helpful. Here’s another tip to get in touch with a domain owner.
One giveaway that a domain owner might be willing to sell his domain name is when a website is taken down. There are many reasons why someone would take down their website, but if they are no longer using the domain name, it might be for sale, whether it’s listed anywhere or not.
If the Whois information isn’t public or is outdated, you can use a tool like Screenshots.com to find more accurate contact information. Many websites list their email and/or phone number in the footer or home page, and if you search for the previous iteration of a website, you may be able to locate contact information for the owner. The domain name might resolve to a parked page or a registrar landing page, but Screenshots.com can be helpful to you to find valuable information.
Another tool you can use for this is Archive.org. I prefer to use Screenshots.com though because they support my blog and are a part of the domain industry.