Buyers can be Sellers and Sellers can become Buyers. A law firm cannot participate. Private - Some Hubs act as extranets or VPNs that connect certain suppliers and buyers together. Many Industry Sponsored Hubs use the private model. Forwards or Reverse Most buying in B2B is Forwards.
Contractor calls the concrete company to get a quote on buying concrete.
Let's call this purchasing initiation Forwards. The Web has enabled Reverse purchasing. Let me illustrate. A Forwards Auction involves one seller and multiple buyers. The seller determines the rules, then the potential buyers bid up the price until the highest price at closing wins the bid.