Well this is one of the things I think we need to find an aggreable sollution for. First off, I think that most of the stuff ending up in the BitzBox is going to be regular bits and pieces. In that area I would expect trades to be tit-for-tat.
One way of handeling anything "more coloufull" would be to have the broker contacting the Stakeholder posting it, to agree upon a fiar exchange beforehand.
Look at the deal we did recently, essentialy you where looking for some specific items, but at the moment you need to post and hope that someone reacts, with the BitzBox up you could browse it, and maybe find what you like.
Part of the stuff you wanted was Special or even rare Rare, so we had mailcontact to confirm the trade, and as a BitzBox stakeholder I am not looking for specific trades (when I would, I´d become a broker too for that!). As my aim was building up more stock (for the moment) we simply agreed that any material would do.
Examples:
There are several strips of goblin wolfriders in the box, you´d indicate the ones you would want, and send an equivalent number of strips back.
You are one short on a chariot bit, which is in the BitzBox, trade it for just about anything else (strip,clip or bitz)
The BitzBox contains around 20 squig°clips you want, to create some squighordes for your O&G,. They are marked a Special, as it is not an out-of-a-blister component. An equivalent number of strips. bitz and clips would be in order, and you´d rumage around you own spares to see if you can find anything special to put in
There is a miniature of Valen in the BitzBox, which I guess would be at the least a rare. You want to grab it before anyone else does, so you phone therelevant stakeholder to ask the price

Well, maybe not that, but consider what you would be wolling to trade for it. There is one stipulation that I would personaly attach to THAT specific deal, and that is that it would NOT find its wat into the "market", but would be proudly presented (painted and based) on the forums within a certain number of weeks.......