But I get what you're saying, you get buyers that want to buy stuff from merchants, they deposit some amount of money with you, then the merchants can go in to a 'reverse auction' (that's what this model is called), the winner is the one that puts in the lowest bid.
You really should make such stuff clear up front when you ask for advice.
This is yet another kettle of fish, with its own associated problems.
You really don't want to be in between here, after all the chances of rip-offs increase enormously, or you will have to go through a lengthy accreditation phase with each merchant where you verify that they can deliver the goods.
In a normal auction the bigger problem is the buyers not being able to pay up.
If there is a performance related issue (such as work for hire) then you have even more things to contend with, such as deadline management, fallbacks and so on.