Something else that helps is to look at the item and ask yourself, "two years from now, am I going to look back and wish I hadn't ever sold this __________? Or, will I even remember having it at all?". When I ask myself that, it's almost always available for sale immediately after.
After 10 years of being in the reselling market, there are only a few items that I now regret selling...and I don't even regret them that much.
One was a set of Craftsman drill bits, and the other was an unusual set of Arvin patio chairs. My regret about the drill bits is that I sold them waaaay too cheap, and really, even though the lawn chairs were cool, they were too old to use. They wouldn't have changed my life a bit...not one iota.
The $100 we got from the above items went directly towards paying down our mortgage. That is $100 (plus interest paid to a bank) that I'll never have to work for again.