In an open bidding system where everyone can see everything going on, it's almost impossible to collude to either obtain a desired result or to deny a desired result. Too many people to create that kind of influence.
Anyways, the upshot is that I agree that it should be open. Why not see who's bidding and who won, and for how much. Almost any other auction system operates this way. And besides, it's more fun and interesting.
OK fine, a silent or closed auction system does not, but how would you implement that in this game? Game's meant to be social. A silent auction system would be what? You put up what you want for sale, don't ask how much you want for it, then keep hoping that a quiet bidding war drives the price up? OK, that can work mechanically, but it sure flies in the face of the 'social' ethos behind the game. And it's boring...