If you are really jazzed about a particular domain name that isn’t for sale, you should do your best to keep in touch with the domain owner in the event that circumstances change and the domain name goes on the market.
I’ve been looking to buy a particular domain name for several years. It’s not an amazing domain name, but it’s a pretty exceptional category defining domain name, and there are thousands of professionals in the field. The domain name has never been developed (or parked), and it is owned by an end user who thinks he will use it eventually, although it appears unlikely.
Every few months, I send the guy a