I do believe it has something to do with Hero Editors etc. programs to change your stats. though I do believe there will be a blackmarket, or boards possibly here for the selling of items in Diablo III
There is a thing called open battlenet
and just normal battlenet
Open you can use characters you made in single player which allows you to bring hacked items into the game.
and normal battlenet you have to create a new account which is serverside making it impossible to hack, there will be occasional bugs/glitches people will abuse but I suspect they will be fixed.
Diablo II used to have a big market for battlenet, and it is still quite large.