Dealer: In a trade between two registered Market Participants (MP), only the sell side reports the trade. Auction: In a trade between two member firms, only the sell side reports the trade. Dealer: In a trade between a registered MP and a non-registered MP (Market Maker not registered in a particular stock, an ECN, etc.), the registered MP reports the trade as a buy or sell. Auction: Trading can occur ONLY between two member firms. (Thus, a buy is never reported.)