A consignment house is a reverse AH, specifically designed for crafter/Fieldcraft to pick and choose their jobs instead of selling their work.
Basically it works like this:
A player goes up to the consignment board, browses recipies, repairs, and melds in an interface similar to that of the AH.
When he finds the item he wants he selects it, and places with it the amount of money he's willing to pay for the item, and the quantity he wants (if stackable) through his retainer. If the job is a meld or item repair, the specific item is also deposited.
Later, the crafter or fieldcrafter can come up, and using an interface that lets them pick the DoL/DoH set they are looking for work in, they see what other people want, and what they are willing to take for that job.