I actually have always disliked Cheapshooters signature line. Its shortsighted and makes assumptions. I can almost bet he would have made at least some of the trades I made, and sold for the some of the $$$ that I got.
My collection is what it is because I did not keep everything. When that good gun comes along, and you dont have the money, because you decided to keep a rusted reblued model 12 shotgun, or a cut down 357 S&W, or whatever, you will see the error in your ways. I don't regret really any of my sales or trades, but I also did it right. If his signature said "be careful when selling or trading your firearms" I would be ok with it, because I am careful and coincidentally, happy with what I've done. I look at my collection as a shrub, and so occasionally I trim it up, but that makes it look better, puts money in my pocket, and allows for new branches to grow. As long as you buy low and sell high, you can never go wrong.
When people say they regret a trade or a sale, its usually because they didn't know what they had, they made a hasty decision, they needed the money for some reason, or some othe similar reason. If you sell and trade right, its often better than keeping the gun.