Based the idea that you are purchasing items from fellow players & not off static in game vendors.
When players buy gold/coin/credits in game they have the ability to buy things that they normally would not be able to. This devalues the play time of others who work to get their gold/coin/credits. In essence, they work a week to buy a special item, but can not afford it, as gold/coin/credit buyers are able to push the cost of items up. The prices of these items are artificially higher. Artificial in the sense, that it is outside of game factors that are causing prices to go up.
If you want specific examples, then go explore the economies of all the games before SWtOR. (EQ anyone?)