Any user can bid in a regular auction. The minimum bid is 1 coin. There are two stages to an auction.
Normal Bid Stage: This is the first phase of an auction where anyone can bid to a maximum of 20 coins above the last bid. Typically, in an hour long auction, this period lasts for 50 minutes.
One Coin Bid Stage: Only users that have bid in the Normal Bid Stage can bid in the One Coin Biding Stage. Users can no longer choose the bid here. Each bid is accepted and registers a bid one coin higher than the current leader. Usually this is the last 10 minutes of an hour long auction. There is a 30 second shot clock that operates here.
The 30 Second Shot Clock: Immediately commencing from when the One Coin Bid Stage starts, there is 30 seconds to make a bid above the current leader. If there is no bid made within that 30 seconds, the current leader wins the auction. Each time there is a bid within those 30 seconds from any user, the clock gets reset to 30 seconds. This will continue until the clock runs down to 0, or until the auction ends at the chosen time. Once the auction ends, the last highest bidder wins the auction.