First, Justin, remember my rule #1 (I don't share my rules, so don't repeat it), you'll then understand why I've acquired so many over time. Then realize that everything is relative. I love to trade knives, primarily when the trade is for a knife I want more than the one I'm trading away. I will always regret trading that knife away, considering rule #1.
As far as selling is concerned, I sell when I'm offered a deal I can't refuse. Not so much in terms of percentage return, though. A 100% return on a $500 knife is $500. Why sell something I really, really like, just to make $500? . Add at least a zero to that, then multiply it by something. That's when it becomes the time to sell, if and when approached. And it primarily causes me to keep what I buy, unless someone is crazy or rich enough to pay the piper. I am a collector, not a dealer.
But that's me.