Reverse auctions (Dutch auctions) are concerned with finding the lowest price for a product or service. It's a technique well suited to commodity purchases, such as buying 2,000 PCs of a specific type. But for more complex products and services, such as choosing a health insurance provider, price is not the only factor. In such cases, using an RFx is generally regarded as the best way to evaluate and select a supplier.
PostRFP does not support Reverse Auctions.
See wikipedia auction page for a good discussion of auction types.