"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.

[Brandish]

I don't remember or I forgot there was such limit
"Only one faction per realm on PvP realms"

This is how it was back then. This is why I have a 6th account, to send love notes to the other faction.

Also fairly certain the PvE servers were also restricted back in the day, might not have been, I was mostly on a PvP realm.

[Wizeowel]

Also fairly certain the PvE servers were also restricted back in the day, might not have been, I was mostly on a PvP realm.

I played alliance on a PvE server but had a troll warrior twink because low level horde were so much more organised in WSG. This was before battlegroups so pre 1.12.

[Tin]

PvE servers were not restricted - but you could not sell and them buy on the neutral auction house - on same account.