Title says it all : user sets a buy price on an SDB, sets an amount of currants to spend and then passively lets it fill up as users sell to it.
This would probably have to be done with a separate SDB than one you are selling from.
My vision is this:
Snotrocket McMufflin needs sparkly, but sucks at mining. Snotrocket has currants though. So he sets out a new SDB in his tower, sticks a piece of sparkly in it to let it know thats what he wants, sets a BUY price and then allocates some cash from his pocket. The max cash allocated can fill up the SDB all the way if he so chooses.
Princess Harfs-a-lot then comes along and sells her hard won sparkly and takes all of Snotrocket McMufflin's cash and runs off to wisely spend them on cubimal boxes.
Teh end.