For you sports fans:
If you caught a history making ball (puck etc.) in the stands would you barter it back to the player? Try and sell it on the open market or keep it? This story sets out the dilemma. In this case they offered the guy a lot of stuff, and a signed jersey etc by a future guaranteed hall of famer which is pretty rare in and of itself.[sports.yahoo.com]
After reading the article, I didnt know you could negotiate and I would have taken the signed jersey and Cabrera stuff
I don't know about other sports but in football (soccer) or rugby I think it unlikely that a spectator would have the opportunity to keep the ball.
In baseball, hockey, and football spectators are allowed to keep the ball in the US. Basketball it just doesn't happen.