First of all, we work with any service (Twitter, Facebook, even bulletin boards) so it isn't just CL - though admittedly it works well there. BidKat just provides additional information about an auction like many people already add to the CL listings.

We don't do payment processing between parties and people don't pay us to create an auction. We also are not a competing listing service, we offer no ability to search our auctions - they have to be advertised separately. Hopefully these things differentiate us enough.