In addition, people want it both way. They want account wide mounts only if they have mounts on alts but do not want mounts they already possess to be sold on this black market AH. Either have both or have neither I would say.
I can't see a huge issue with a decent drop rate RNG mount being put up /extremely/ rarely for bid - However the above issue of "it'll most likely be people who bot" winning it sours the idea.
And putting T3 in there? They couldn't have just brought back the actual instance?
Bots will get banned which is always awesome btw. Also most bots are compromised accounts so they don't care about this AH that sells "useless" vanity BOP items that have no effect on your game play at all, also no real life profits are to be made from this black market - it's all gold that just disappears.
Typically a server will have maybe a handful of gold tycoons or barons, once they get theirs it goes to the next in line and so on and so on. It all depends on server size of course, some servers are pretty fair competition unfortunately it's not going to be high pop servers (for the black market anyways).
I don't feel bad for people who have tcg mounts on alts or whatever, when you have too much gold you tend to just throw it at anything even a level 20 alt with a spectral tiger (example). Some people enjoy camping the auction house all day and being "Wal-Mart" but that's another story I despise the auction house system that's just OCD mode. The only people I'll feel bad for is those who spend real life money otherwise auction house campers / gold makers - sorry don't feel sorry for you.
One possibility I see with TCG mounts and account wide is that people might get the BOE versions for how many extra they have of them? If not then someone could make this suggestion on beta otherwise tough luck.
The only point I will agree with you is bringing back Naxx 40 and any other old instance like Old ZG or mounts that were removed , etc. , but seeing T3 on the black market is nice. I could care little for Xmogging or anything but for those who do happy hunting and greet your new gold sink with open arms.