dont know if this is already known to people but works for me and other people to.
walk into the ah and head to te auctioneer that is furthest to the left, then look to your left and move into the corner, now you in the corner as far as you can move back a bit NOT ALOT just a tap of walking backwards will do, then jump forward, (this may take a few attempts to get used to).
if you succesded it will look like you are off the ground noticably, so now you are about 1/3 up the wall.
turn about 45 degrese to your left and jump up then forward to gain more height, after a few more jumps you will be on the wall at side of ah, walk forward along the wall untill you get stopped by invisible barrier that stops u getting behind the Auction house, move back a bit and line youself up with the path so it looks like you are blinking further along the path (not into the wall), if succesfull you will be behind the AH if not u will be in same spot that you origonaly were for more attempts, too me a few attempts to get the hang of this, done it loads of times as many other people have, i will release a video of how to do it soon if i can and i know a couple of more orgrimmar exploits.
this will guarantee a few whispers asking u "WTH how u do that kek"
hf and yeah u can still speak to Auctioneers behind the ah where they stand and even the guards at back of AH.
good luck have fun, more posting soon