"Only one faction per realm on PvP realms" and other just announced limits

[Apatheist]

I would imagine it only being harder in classic.

I kept a max level alt at all of the neutral AH's from classic all the way through Wrath. With the right addons and macros you can just mash buttons to list, search and buy with your alt pretty much instantly. You can generally tell when there are bots scanning the AH since 90% of the time they're low level characters with weird names. Even if you do lose an item here and there it's trivial compared to the amount of gold you can make overall.

Faction specific rewards sell pretty high. In particular I remember deleting thousands of gold on my horde alt because I didn't notice for ages that it was gold capped from selling alliance argent tournament pets in Wrath.