I've done this several times and usually has been fine. It can go either way and depends on the people. Personally, as a seller when I'm ready to sell, I'm ready. Time to move on and if I want the game back, I expect to pay what they want. But I did appreciate when the friends mention they're ready to sell and asked if I mind or was interested. I guess it depends on you and your friends more than anything. I've pissed off a few friends too (non-intentionally) by not offering them a machine I wanted to sell or had someone local stop over and buy when I wasn't even planning to sell. Neither time was intentional but shit happens.
Short version, it's your machine while you have it and do what you want. Once it's gone, it's out of your hands.
Quoted from Spyderturbo007:I don't see any problem with it at all. I bought a BC from dmacy. He asked if I would let him know if I ever decided to sell it. He's my friend, so you can be damn sure he gets first crack at it. I can see how it could be an issue if the sale price doesn't adjust with the market.
That guy was a jerk and you likely overpaid. BTW, would you sell it to me for $500? 
