DiabloFans.com has cranked out an editorial that provides a brief analysis of the player economies that exist in Diablo and Diablo II, as well as what we might expect from Diablo III's player economy with the added features brought by Battle.net 2.0. Developer quotes are thrown in for good measure, too. A snip:
The biggest problems of the economy of Diablo II were that it is not a proper free market, because there is no dynamic supply, and that it had no efficient means of in-game trading besides flooding the chat channels. Supply was artificially being created by drop chances and there were no means of having it follow demand. Think of the system as if demand was developed in a free market but supply was decided beforehand much like in the (failed) planned economies that existed in former Socialist countries such as the former Soviet Union.
Another huge issue was that because items that could be found were infinite and only limited by the time that had passed from the last ladder reset, the market was experiencing some heavy fluctuation and deflation over time. Heavy duping ran rampant, which ended up in exponentially increasing the supply of certain items such as high runes. It was certainly not fine-tuned by Blizzard to be a working economy.
The most obvious concept would be an in-game Auction House operating via NPC(s), or a sort of Battle.net 2.0 trading market within the game. This is purely speculative but it also could be like a trading forum integrated to Diablo III and you had a list of all the items that were on your characters available for you to browse, so you would be able to show them to other traders with ease.