With the recent promotion over I have actually been going over and reading all the questions we have amassed and I have found that there are a large collection of questions specifically about the auction house. A quick query for "diablo-3 auction" returned 162 results with a fair share of closed questions and some that mention it in an off-handed (like the barbarian build one).
A quick poke in the channel of those who were active pushed this thought forward enough to tag a single question with the new tag so I could get it all set up and then this post here so people could give some thoughts back on pushing this tag out to appropriate questions. I do not think all 162 need this tag but I would be surprised if we did not end up with a quantity rivaling the current class sub-tags.
So the new tag would be diablo-3-auction to keep its size in check. I have put in a very short and sweet wiki description and invite the community to poke it around until it is fitting or just state that this tag would not be useful.