Bids are placed in ETH.
Bidding is incremental and needs to be at minimum 1% above the current highest bid.
The bidder with the highest bid can not withdraw their bids. All the other participants can withdraw their bid whenever they want.
There is no need to withdraw the previous bid to place a new bid. In the occasion a user has been outbid, they can place another bid with the total amount they are willing to bid. 
The ensuing transaction will be only for the amount to add to the previous bid to match the new bid price.