When we were out at Blizzard on Tuesday, they told us players would be able to buy and sell items for real money , and the game will be online only . There's another part to that, closely related: not only will mods not be supported, they're specifically forbidden by Blizzard. Official quote below.
Some games don't have official mod support but can still be tweaked and enhanced by the community. Others, like World of Warcraft, don't let you change the fundamental mechanics, but give you free reign over rearranging the interface and searching the auction house with mods. The official FAQ provided to us by Blizzard makes it Diablo 3's policy pretty clear:
Having played the game, I'm not desperate to re-arrange its interface or scrape its auction house for the best prices. And it's no surprise they don't want you modding the online game: it would rather undermine the item market. But I do think it's a shame there's not an offline mode we can tinker with and create interesting things for. StarCraft 2 went out of its way to embrace that kind of creativity, with an editor so flexible you can make new games in it.