“I might need it someday” is really a curse for getting rid of stuff.
Treehugger has a typically fiddly Lifehacker points to a fiddly way of overcoming the urge to hoard stuff “just in case,” but I think their this method would likely result in keeping a mental inventory of what you’ve sold vs. what you’re replacing with the funds. That’s a link to the stuff that should be severed right away – out of sight is not out of mind, and snipping the mental link is probably even more important than physically removing the item from your life.
Here’s an easier way that helps me. When I legitimately think I might need something in the future, but don’t need it now, I pitch it to myself as a “minimize it forward” event. I give it away, usually via freecycle, or sell it, free and clear of any special accounting. Someone else uses it and extracts value from it. If I need it again, there is a decent chance I can find it within a few days on freecycle, craigslist, or a garage sale. If not, I get creative, do without, or buy a replacement. The key thing is to try and keep everything in play – ideally someone should be getting value out of every item at all times. If I’m sitting on something “just in case,” I’m basically preventing it from being used, and probably causing another one to be created for someone who needs it.
Minimizing is good for the world, not just the individual, so keep what you really use, and put the rest in play.