This reminds me of the time a few friends and I were playing a modded minecraft server and destroyed the market and entered into a post scarcity economy. The mods were different, but we took over so fast that every admin on the server was investigating us for hacking (eventually we were cleared).
The server we were on was much smaller, and while there was an economy, it was never to the point that B would buy from A just to resell at a higher price than A was selling.
One game that fascinates me with its economy is Path of Exile. It tried to do away with gold, and in doing so gave evidence that there will always be some sort of currency. Granted, I think this was already shown in Diablo 2 when, because of the gold cap being far below the value of the best items, a second organic currency economy was born trading in runes (I think it was runes, its been a long time).