The buyer is an auction and determine requirements, vendors come to bid. Vendors that can offer the lowest sales price as the winner of the auction. For the public sector are often used.
Buyer is starting an auction and determine the need for vendors to bid race. Sellers who can offer the lowest price wins the auction. Government is often used for this.
The buyer is the auction and the need for the seller to offer competing. The seller can offer to sell the lowest wins the auction. For the public is often used this .