One of our most popular selling rifles is the M&P Sport, which comes with a Rear Magpul MBUS. Up here in ND probably 75% or better AR style rifles that we sell leave with some sort of scope on it. Putting an actual scope on makes it impossible(sort of) to leave the sight on so I always offer the person buying the rifle if they would sell the sight for $20, almost every one of them sells it to me.
I then mark it $30 and throw it in our closeout bin. I have the rear sight in the Magpul packaging hanging on the wall for $40-43 usually so it is a win win for all three people that come in to possession of the sight.
The buyer of the gun gets $20 for something he doesn't care about and will never use, I make $10 and the buyer gets a brand new Magpul rear sight minus the packaging for $13 less than "new"