I have a list of cards that I am always willing to buy and prices I would pay for each. Is there any chance that you would be willing to create a new section in the Voodoo Auction where a prospective buyer can anonymously request specific voodoo and can post the max credits he is willing to pay. Then he can get an event notification when a request has gotten an offer. It should maybe have boxes to indicate whether the buyer is looking for the lowest price or just immediate availability and whether this request should stay up until manually taken down (e.g. player wants many of the same card and will likely buy all that are offered. This may drive up prices.) or until a developer-imposed time limit has been reached or if he is just looking for a one-time purchase. If the player doesn't have the means to pay his max price then his request should be blocked. If it is posted, his price in credits should be withdrawn from what he has until the request is removed.
I assume the gears are turning and that you can see how this would entice players to buy credits so that their desired voodoo can get to their inventory quickly.