Its actually a different kind of auction where the buyers role and the sellers role are switched. Maybe I could explain the business model to you in detail over here: http://etherpad.com/FhFO67ihv2 ?
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.
For instance, if there are no 'takers' then you end up having to refund but the charges are gone.
A hard to catch fraud angle here is a person laundering money putting charges on cards for deposits, then having a 'white' merchant receive the deposits with ridiculously low bids.
We have sort of solved that issue. The only issue left to be solved is authorizing a deposit against the buyers account before the auction, and then transfering this amount to the merchant at the end of the auction. This deposit is not only going to be part of the price of the merchandise. Any idea on how this model could be improved?
Longer terms you will have to simply charge the account and then do a refund.
So this very much depends on the timing of your auctions.