how to play You're Bluffing

About You're Bluffing!, from BoardGameGeek:

Classic bidding and bluffing game. The deck consists of sets of farm animals, 4 cards per animal. Players auction off the top animal of the deck, or try to complete their set by trading with another player. Highest bid wins, but auctioneer always has the choice to buy the animal for the same price as the highest bid. Trading is a blind bidding system: both players place their bid face down on the table and reveal them simultaneously to each other. High bidder gets the other player's animal. Both players swap their bid. It's best to bid just a little bit more than your opponent, but just how much did he bid? reimplemented by: Kuhhandel Master (2009)