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.

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?