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

[MiRai]

Auction houses aren't centralized in vanilla. You can only view items listed at the particular auction house you're at so you can just kill anybody standing near the auctioneer before you list your goods if you're worried.

Same-Faction auction houses, as well as trade chat, were merged in patch 1.9.

Players will now be able to buy and sell goods with greater effectiveness using the Linked Auction House system. Auction Houses in Orgrimmar, Undercity, and Thunder Bluff will now share the same pool of Horde player-created auctions, and Alliance players will find the same to be true when visiting Ironforge, Stormwind City, and Darnassus Auction Houses. This system has been expanded to support the neutral Auction Houses as well. Tanaris, Everlook, and Booty Bay will all be linked for players of both factions to access. In addition, the "Looking for Group" and "Trade" channels have been unified among the corresponding cities, meaning, for example, that you can trade your goods or look for groups in Ironforge while in Stormwind.