I have owned ... for years ... the .net version of a domain name. The .com version was already taken but I would have preferred it.
Last month I received an email from the owner of the .com domain name ... offering to sell it to me for $89.00 USD. It was out of my budget range ... but I decided to check on the domain name status anyway. Apparently, the domain name had lapsed and was available to be registered by anyone. So I did ... through my usual registrar.
Moral of the story ... if you want to sell the domain names ... DON'T LET THE REGISTRATION LAPSE.
Peter