The Diablo series in general isn't heavily multiplayer focussed but you can play with your friends on servers. It's not an MMO, but it does have some multiplayer. There are also 'duels' which is apparently a feature in D3. The major reason(s) why they're introducing this system is 1: there are third party sites out there selling diablo 2 items(diablo 1 items too?) and those sites are unsupported, unreliable, and frankly, risky. Blizz wants to discourage people from using them, and they'd rather do that by providing a viable alternative. So they're making an official auction house for people to trade in. Sounds good to me. The number 2: reason, and this is my own speculation, they're basically using this as a 'test case' to see how well the system will work with real currency, and if it does well, they may add it to their other games(WoW). How they would implement it in WoW is totally speculative right now, but I imagine it could function similarly with very little in the way of alterations. BoE's sellable for real money? Check! BoP's tradeable for real currency? Nope! Can't sell them on the existing AH. Selling ingame gold for real money? Ah this is where it gets interesting. There are quite a few players now, sitting on gold-capped toons. Will they be able to turn that into real-world money? Who knows?