Any guild with more than 50 members (I think that's the number) can set up a guild store, where guild members can buy and sell within the guild. It's not an auction house; the seller sets a fixed price.
A guild that controls a keep or a resource (like a farm) in Cyrodiil can open their guild store to the public by talking to the quartermaster there. Then other players in the same alliance can buy and sell at the guild store.
Should also be noted that you can belong to up to five guilds; players are already planning "merchant guilds" with commerce as the main focus, and you can belong to one or more of those as well as your main gameplay guild(s).
And, of course, you can set up trades individually with players via chat.