Actually, Square-Enix has taken the lesson. In fact, they did it a month ago. We've seen a lot of it of FFXI, and they did take action. Within a week of that change, we saw a dramatic drop in characters who were farming gil (game money) for real cash (a.k.a. gillsellers). I've heard that some of the sites have been shut down but I didn't really know where they were to begin with so I can't confirm it. They've made some more minor revisions as a follow-up, but I think their first move was the most effective. There were areas that I would sometimes have trouble walking through because it was so crowded with gilsellers and bots, and now when I go there it's usually almost (if not completely) empty. There are still hymns of praise being sung by players about this decision. It sounds a little silly to people who don't play, but the gilsellers have made far-reaching impacts on the gameplay and despite being banned still manage to do so. FFXI (as well as any other MMORPG, i'm sure) has an interesting player-driven economy. Gilsellers upset the balance of that economy by monopolizing access to certain items and railing the price so they have more gil to sell. They upset the gameplay by sweeping through areas and killing everything on their runs, making it pointless for anyone who is there after a monster or trying to gain experience points. Kudos to both companies for their decision and any future ones that follow suit.
As much as many people would love to see the same of D2, I don't think that's ever going to be a reality. It would be nice, but at this point it really isn't fesible. This stuff is a huge business, which is kinda funny when you consider that the vast majority of players want nothing to do with people who do this. I've seen char's kicked out of parties and occasionally run out of a zone because they were a gilseller. But some of them don't have a choice but to be there. There are companies out there whose sole income is getting items and selling them for money. They just have a bank of computers, a few employees that get paid peanuts to work shifts on the characters to keep the flow of items and money constant. It's kinda frightening. Square-Enix (and Blizzard, at least in the case of WoW) took the initiative to curb this practice but in the case of D2 the damage has already been done and is far too extensive. Even if Blizzard filed a lawsuit on every site that sells the items and/or gold and got them shut down immediately, the market has already been irrevocably changed by their activity. They've been there for so long now, and become so entrenched that even in their absence I doubt prices would retern to normal.
Oh, and for those who are interested: my name is Judaas and I'm on Shiva.