What does Blizzard think of this activity?
In the case of World of WarCraft (WoW) is strictly prohibited by the Terms of Use.

"You may not exploit World of Warcraft for any commercial purpose, including, but not limited to, performing "power leveling" services to other users of World of Warcraft for "real" money.

Blizzard Entertainment does not recognize any property claims outside of World of Warcraft or the purported "sale" in the "real world" of anything related to World of Warcraft. Accordingly, you may not sell items for "real" money or trade items for things of value outside of World of Warcraft."
-World of Warcraft Terms of use

It is not unsurprising then that Blizzard has banished over a thousand WoW Gold users from the lands of Azeroth a few weeks ago.

"Over the recent weeks we have been investigating the activities of certain individuals who have been farming gold in order to sell it in exchange for real world currency. After researching the situation, we have issued permanent suspensions to over one thousand accounts that have been engaging in this practice. We do not condone such actions and will take decisive action as they are against our policy and damage the game economy as a whole. We will continue researching this matter." - Blizzards press statement