I have two arguments *for* flipping (buying low and selling high without changing anything in the process) but they really aren't that relevant specifically for HC's player-run economy. 1. It keeps the system liquid so that sellers always have a buyer and buyers (hopefully) always have a seller. I'd argue against myself on this point though, because supply is so extraordinarily easy to come by on HC that for the most part it is much more profitable just to make more of something rather than flip it and take the 10% transaction tax. 2. It's a price-finding system, so that people will bid creep and undersell in market PvP (lol) until they reach a competitive equilibrium point, which in theory is good for both buyers and sellers. I just don't see that much economic PvP though, so a wide bid-offer spread stays wide and the market makers get rich because no one is willing to compete with them.
So yeah, for the most part, when I flip (which I really don't do very often. There is usually some crafting and converting involved as well.) I'm mainly just doing it for egregious profit rather than for the good of the economy.
This, this is so true it brings a tear to my eye. Nicely done again, @Ukase. The only reason flipping works (or any variation of buying things on the cheap) is that sellers dump their goods in order to insta-sell. And then they complain that they aren't getting enough for them, or that they are not going to the people who really need them.