You can bid on an item by straight bidding or automatic bidding (automatic bidding is also referred to as "proxy bidding").

• Straight bidding:
  Enter an amount higher than the current winning bid (displayed on the auction page as "Current Bid"), and click the "Bid this exact amount" radio button below the bid you just entered. Then click the "preview bid" button. The system immediately places your bid at the exact amount you entered.
  For example, if the current high bid is $1.00, and you entered a straight bid of $15.00, you're now the current high bidder at $15.00.
• Automatic bidding:
  Automatic bidding places bids on your behalf at the lowest possible increments. Simply enter your maximum bid (the highest amount you are willing to pay), and click the "Bid up to this amount on my behalf" button.
  The goal of automatic bidding is for you to win the auction at the lowest possible price. Your maximum bid is only placed when another bidder also bids up to that amount or when the reserve price is equal to or greater than your maximum bid.
  Bids increase based on the bid increment set for the auction. Therefore, the current bid equals the former bid plus the bid increment.
  Example 1: An item's current bid is $3.00, and the bid increment is $0.10. Jane chooses automatic bidding and enters a maximum bid of $6.00, so Jane becomes the high bidder and the current bid reads $3.10.
  Example 2: An item's start price is $3.00. John chooses automatic bidding and enters a maximum bid of $7.00. John is the current high bidder at $3.00. Then Jane enters a maximum bid of $6.00. The current bid is now $6.10 since John has a higher maximum bid.

Learn more about bid increments.

How does a buy price affect automatic bidding?

It's important to remember that when you bid the buy price on an auction, you're choosing to win the auction immediately at that exact price. Therefore, when you bid the buy price, you override the automatic bidding process. If you prefer to use automatic bidding when bidding on an auction that has a buy price set, make sure to place your maximum bid below the buy price amount.