Auction - Bidding on an auction is done in specified increments. The user with the highest sucessfull bid "wins" the item. Auctions use Auto Bid or Maximum Bid. The price you enter tis the amount the system wll bid up to for you on your behalf. 

Buy it Now - Some items have a buy now price available meaning that you can buy that available item for the price displayed, without having to wait for the auction to close. Once the quantity is sold, the items are gone. Buy it now overrides bids placed. The transaction that comes through first to us will be the successful winner/s.

Auto Bid/Maximum Bid - An automatic method of bidding that increases your bid to the next lowest price when another user bids on the same item. Your bid is increased until your maximum bid amount is reached or the auction ends.

Reserves - Some items have reserves. The reserve price is the absolute minimum dollar amount that will be accepted for an item. Buyers are notified on the bidding screen after placing their bid if it is below the Reserve price.