Why is the highest bid for a house 0, even though I have made a bid?
Consider the amount you are bidding to be your individual limit, i.e. the maximum amount of gold you are willing to pay in order to rent the house you want. As long as your bid limit is not exceeded you will automatically try to outbid all other bidders by 1 gold piece. As a consequence, you will get a house for 0 gold if there is no other bidder.
Example: You have set your bid limit to 1000 gold. As long as there is no other bidder the bid remains set to 0 gold. Now another player appears on the scene who is prepared to bid up to 500 gold. However, since you have set a higher limit you will automatically outbid him. You are now bidding 500 gold pieces.