Technically, most everyone is a flipper since we sell most pins at some point.

I guess, however, I would consider someone an actual "pin flipper" if they bought a pin with the explicit intention of selling it as quickly as possible at the greatest profit. Some find a great deal, do nothing but re-list it, and make money. Some extensively rebuild them from top to bottom.
Obviously, some people frown on those that buy, do nothing to it, and sell for more.

The truth is, we live in a capitalistic, free market society, and knowledge is power. It isn't "noble" work, but flipping requires knowledge, skill, grunt work, and marketing...all of which are free to anyone willing to apply themselves.
We are all just trying to earn a buck. If soneobe puts forth the effort to flip, I see no problem with it.

This literally happens to everything eventually. It has ravaged the housing market, cars, collectibles, etc.

It is ingrained in our society. Everyone praises it until it costs them money.