Per currently published Flippa user agreement online (#6 specifically), if you list a digital asset for sale via Auction you agree to not advertise or offer to sell the digital asset through any other third party service or make direct approaches to potential purchasers outside Flippa.com unless otherwise authorized by Flippa.com.
Evidently, this restriction does not apply to digital assets as a Classified or Catalog Sale.
Please read the agreement for all details.