Every listing created on eBay has a unique, individual number allocated to it. The number is 12 digits long and can currently be found in the "other item info" box near the top of the listing itself.
In the same way that any auction works on the site. You place your bid, other members generally bid against you and the highest bidder when the auction ends is the winner and the winner has to pay...
No, that is not possible.
An eBay auction can run for 1,3,5,7 or 10 days, but once you have made the selection and submitted your listing, the duration cannot be extended.