So i found this out while useing the new Engi AH: Steam powered Auctionner.
I bought some things then i looked at my mail, Mail from: Alliance Auction House. Im horde so i bought a few other things and it said From the alli AH so i tried adding a seller to my freinds: "This player Must be a Part of your faction".
This will work to my advantage because On my server it is mostly alli pop, so Nobles cards, epic gems and lots of other goodies.
Just thought i should share
~garfieldfan