I see nothing wrong with this whatsoever, and we call it "keeping it within the circle", or network, family, etc.
My Class of 1812 is spoken for by a good friend who actually sold it to me under the deal it would be his when I'm ready to move on.
My only rule is that what I currently have in it is what it gets sold back as, as I've made upgrades and improvements since taking it over, or a number that is fair market price. Obviously everything can be talked out, but I keep a spreadsheet of every penny I put into each game and so it's no mystery where I'm at with it. If he doesn't want it after a few weeks heads up it's ready to be moved, then it goes elsewhere. I am fully committed to the gentlemen's deal.
If you have a good relationship with the friend, communicate and have a gentlemen's pact, go for it. The worst that can happen is they sell it behind your back or don't adhere to the agreement. If that happens you just learned who is and who isn't a trusted friend. Simple as that. It's just a game.
In fact, if I have a rare game to sell, and a person I know wants it, but I know they're the kind of guy who sells on a whim and doesn't like these pacts, then I may not sell to him. I'll first try the contacts I know will run it back by me. Of course this only works if I'm not in an emergency to sell or anything and can hang onto it if I don't want to cut it loose into the wild.