It's always been part of the game. Always.
The first time a seller backed out on a sale from me was in 2002, when I was trying to buy my second game, and a dude with a Space Mission suddenly couldn't sell it "because it's my wife's favorite game!!!"
Of course, at that time, humans interacted in other ways besides online feedback reviews, so I was unable to stretch it out for a month with dueling bad feedbacks and a thread on pinside. Yes, it stung. You could say I was crushed! I REALLY wanted that $400 Space Mission!
But then 2 days later, I found some other game to buy. Turns out there's thousands of games out there.
Life is short. I don't begrudge people for playing the Yelp! game, even if it's not really my cup of tea. But when feedback is used for revenge (which seems to be the motive here), it always starts an obnoxious cycle like this; anybody who ever used eBay back in 1999 should probably know this by now. I don't think feedback is particularly valuable on Pinside, since you can leave it for anybody, at any time, for any reason, even if you haven't bought or sold a game from them. With all the bickering that goes on here I'm surprised the system hasn't seen even more abuse.