A flipper adds no value to the game and just resells it solely for profit; a few hours or days after acquiring the game for a pittance.

The thing is that there are other people who usually see the original ad who are interested in the game and wouldn't just flip it for profit. That's what ticks people off. People who would want to add it to their collection to enjoy and/or fix it up get beaten out by a flipper just by answering the ad first. The flipper adds no value whatsoever, and there's a line of people who would buy it regardless of whether or not the flipper saw it first. A flipper isn't typically bringing a hidden item into the public marketplace. They just grab it from the public marketplace and relist it in the same public marketplace.

People who buy games, fix them, and then resell I have no problem with. They add value to the game, and it's perfectly valid for them to charge for their time and effort in shopping, refurbishing, and/or restoring the game to a plug & play ready state.

Some people buy games to enjoy, then later sell and get something else. That's fine too. They're buying/selling/trading for their personal enjoyment and participating in the hobby.

It's also not uncommon for collectors to sell off games to make room for other stuff. I've been guilty of this. I get a good deal on something, but later lose interest and find some new bauble to chase after and acquire. Most of the time, I basically let those go at a break-even price.