The bidding system has practical problems when buying, but I think it is the one that ensures the most transparency, since anyone in the game with the money can bid. And replying to Shane24, that the time is being postponed is part of any bidding system, perhaps it could be shortened from 3 to 2 minutes, to make it less exhausting.
In your propose (JeriSlouni), two teams could agree to sell a player at very undervalued or overvalued prices. In any case, it would be not very transparent, and if we used as reference the estimates prices, sadly sometimes are ridiculous values.