Diablo II was a game about random drops. You clicked on things until they died and then clicked on all the things they dropped to see if any of the gear was good enough for you to switch out your equipment or if you should simply sell it for gold.

While Diablo III can still be played that way, the addition of an in-game auction house which allows players to easily buy and sell gear complicates the game far more than you might expect from a system for simply buying the new gear you need and selling whatever you don't want to somebody that does.

Let's look to the past for a moment; what economy Diablo II had was largely a black market of shady deals haphazardly assembled by the game's community. It's telling that Diablo II traders used an item, The Stone of Jordan, as a de facto currency rather than the in-game gold (which was practically worthless in player-to-player transactions). Blizzard hadn't really planned for much of a trading community in Diablo II, and it showed.

Diablo III, however, has a highly organized auction house (though I'll be the first to admit the interface needs work) that allows you to search for the stats you need on your gear to quickly and efficiently improve your equipment using the money you've made from playing the game and selling other items you've found.