I have been using this for a while on gems/items/anything else semi valuable.
First, put something on the auction house that you would like sold. Then make a level 1 with a random name and take it to a city. Start posting in trade channel that you are looking for someone that has the amount of gold for the item you are selling and would like to make 25 gold(or something in proportion to the item you are selling).
Eventually you will start getting tells asking what they need to do.
Tell them that you have a character on the other faction and that you really need said item and that you will pay them a certain amount of gold to go put it up on the neutral ah. You should offer a generous amount because most people know that the cut in neutral is worse so just tell them that you are willing to buy it for a lot more
Generally people will buy out your item and go put it in neutral ah. If there are a lot in there tell them you need a few of the item since there are none on the ah.