There doesn't have to be an auction house to have an economy. Once trading is enacted, the game will still be subject to the laws of economics. This can be seen by looking at Diablo 2 and even 3 without the auction house anymore. Inflation in the number of items will lead to a decrease in perceived value of those items, this is a basic principle of supply and demand. People won't have to work as hard, to get items, which means they will get them faster. People will then get bored with what items are available as they will obtain them at a faster rate. More supply of items (from not breaking them down) will lead to a lower demand for the current offerings. Requiring new items to keep people interested in the game. Bungie will then have to make more items faster than if they didn't have trading, which means that they would spend more time on making up and developing items, and less time on other aspects of the game. Again, I'm not saying it's necessarily bad, just a matter of what you want more. More loot, or more activities to do with the loot. Only a limited number of resources Bungie has to put toward development.

Currently there is no trading though, so there is still no player to player in game economy. And the vendors don't suffer from supply/demand or inflation.