I have a confession to make: I'm addicted to purchasing domain names. Whenever I get an idea for a project, I can't stop thinking about how to name it and when I find a good enough name, I have to purchase the domain for it. The result? About a dozen of domains doing nothing. I could just release them, but I feel like some of those can actually be worth something to someone (they do to me) and I have not necessarily given up on creating something with them.
So instead of letting them sit on an ad page for GoDaddy that gives me absolutely no benefit, I created a simple "domains for sale" website (hosted on a subdomain on one of my existing domains, I'm crazy enough to buy another domain for that) and I forward each of them there (the subdomain in question is domainsforsale.mbillard.com). I created the website with Ruby on Rails, because I could create something simple quickly and because I could make use of the excellent heroku hosting which is free for websites with low performance requirements.
What really makes this work though is that each of the domains forward to this page with the name of the domain as a parameter (ex: ...domainsforsale.mbillard.com/?domain=[domain name]). Combined with a Google Analytics profile, I can now track which domains are accessed and how often they are. This helps me know more about which domains might actually be worth something. Clearly stating that a domain is for sale also helps users looking to purchase one of those domains because the contact information is right there, users don't have to go search in a whois databases.
If you have inactive domains that you would be willing to sell, there's no reason not to do this.
Note: There's also an administration section not shown here that I use for managing the domains (in case I sell or buy one).