There aren't a lot of things that you can buy, use for a couple years, and then sell at a small loss, or in some select cases, make a tidy profit. But one of the great things about quality gun and knife gear is that it tends to hold its value pretty well.
Case in point: Project AR-(20)15 is largely being funded by clearing out unused stuff. Projects that ended up going no where, ideas that have been abandoned or gear that I wanted to upgrade anyways. Stuff that was of little/no use and was just sitting there, collecting dust, and that sold off pretty quickly, painlessly and in some cases at a healthy profit over what I paid for it a few years ago. Made enough money to fund the purchase of a very nice BCM upper with minimal out-of-pocket cash.
Should have done that a while ago!
Yes, you can keep stuff around "just in case", put it in caches, and so on, but there's a bit of a false economy to that. It might not cost you anything out of pocket to keep, say, an unused hunting rifle around, but it is an unused resource that can be turned into cash. Sell or trade it in, and a new purchase becomes a lot less painful on the budget.
Sure, in some cases it makes sense to keep extra gear around, but that's not always that case.
So - if you have been eyeballing a new project, go clean out the part bins/drawers/closets and see what shakes out.