I have plenty of reasons for trading as opposed to buying.

1 - Trading can eliminate different opinions in pinball pricing. I've traded games a couple times with retailers. I would never buy the game they had at their retail price, but I was able to trade even on my games.

2 - No down time. There is instant gratification gained in trade that can't be matched when selling a game.

3 - No need for cash. I have my pinball fund ready and waiting for when a deal comes up, so why sell. I usually only sell when I have a replacement already lined up or I have run out of space.

4 - Trading gives you more options. Simple fact is there will always be games only available via trade with like-minded collectors.