If a bid is placed in the last several minutes of an auction, the auction end time will extend for an additional several minutes. This is called "Popcorn Bidding" and gives all bidders an equal chance of winning an auction if a last minute bid is placed.
The exact number of minutes may vary from seller to seller. Please visit each site's FAQ under "Bidding" and click "Why Do Auctions Extend At The End?" for each site's specific policy.
Popcorn Bidding is used to simulate a live auction and prevents other bidders from "sniping" an auction at the last second.