Basically it can work one of two ways ... first the source (perhaps Prim or perhaps someone else with access to a box) picks up the box. At that point, two things can happen -
1. the source opens the box and all the bags, divides the minifigures into 3 complete sets and sends them out to people on the board who are interested for a reasonable price (shipping + 1/3 of the box's cost I would say, perhaps a bit more for gas and time)
OR
2. the complete unopened box can be sold to one of us on the board from the source and then we take care of all the dividing, shipping, etc.
The easiest is probably for the source (the one who has easy access to a complete box) to grab their figures and then split and ship the rest to interested parties.
But if someone like Prim has access to a 2nd box and doesn't want to deal with splitting the figures up and working out shipping, I would be fine with buying a complete box from her, paying the shipping cost and then unbagging and splitting up the sets while factoring in the 1/3 of a box cost plus shipping (adding 1/3 of the shipping cost I would pay Prim + whatever the cost is to ship the smaller set out).
I do quite a bit of selling on Amazon and eBay so I have plenty of boxes, a scale, etc. It would be really easy for me to take care of that aspect, I would just need a source for the box itself. Otherwise, as long as everyone is ok with receiving their minifigures opened (let's be real, we can't divide the sets w/o opening the bags) then I think it would be easy to split a box 3 or 4 ways because shipping costs for the divided sets should be low.