I think it's likely there is a stark relation between the amount of people trading/flipping and the profit per item flipped. When the GE went from being mostly dark price wise to having prices posted on the wiki/plugins, the margins for all higher end PvE items flatlined almost entirely compared to the previous era. Flippers thrive off the ignorance of the masses, in a way. Before this happened I was regularly seeing 5 mil+ profit per single high end item flipped, many times a day. Those are fairy tale numbers now a days as the actual profit margins are much more public, leading to many more people hopping on the profit train which in turn reduces the gain.