I started out collecting cart only and have since moved on to CIB. The process of converting my NES collection has produced a good number of doubles in my collection. They are all technically for sale but in reality they continue to sit in my basement and I use them as the playing copy of that game.
Unless…it’s one of the games I still have from childhood. Then all bets are off. Dragon Warrior for example, I’ve got my original copy, a playing copy, my CIB copy, my famicom copy, and I‘d be thrilled to have a sealed copy.