If you own a .COM domain and you want to sell it, here's a tip: Search for existing owners of .NET, .ORG, and other TLD's too see if they are interested. Often, if it's an end user, they'll be interested in upgrading to the .COM.
Uniweb has a tool you can use to search about 70 different TLDs. It's quick, but not always reliable (I saw a few false positives in my testing).
Happy Domain Hunting!