I no longer actively play the AH since things have slowed down prior to MoP launch, but I was a big contender on my server. In order to play the AH you need gold to make gold. Start small. Sell of random BoE's, farm embersilk cloth. Since you have a DK -- giving him tailoring is great idea. You get the cloth scavenging ability and there is a great location in Deepholm where the mobs are practically insta-respawn in decent size packs. They drop large amounts of cloth and boe greens/blues. Otherwise Jewelcrafting and Enchanting are a great combination. Just do your research, find your common gem cuts and enchants and post them on the AH for reasonable prices. They will always sell fast. Players are continuously making new alts and are looking to gear up. Just as a side note, if you take the tailoring consideration pair it with enchanting to d/e all those greens you'll be finding. Sell enchanting scrolls and the raw materials themselves.
