I think the IRS or Blizzard would have a different slant on this if it was meant as a casual thing, i.e., the occasional sale on ebay of a rune or uber item - irregular income. If, however, this was meant as a full-time occupation then the IRS would consider it earned income. It would mean a lot of paperwork to do legit: tax forms, bank statements, etc.

This actually poses a good question: who owns, in a legal sense, the equipment I use? Me or Blizzard? I don't really know. Medieval Bob's comments are to the point however, there is no current rule against it that I know of.