To end an auction early, go to
http://signin.ebay.com/aw-cgi/eBayISAPI.dll?SignIn&UsingSSL=0&pUserId=&ru=http%3A%2F%2Fcgi3.ebay.com%2Faw-cgi%2FeBayISAPI.dll%3FEndingMyAuction%26pass%3D%7B_pass_%7D%26userid%3D&pp=pass&pageType=562&i1=0
If that doesn't work from this page, from eBay's main page click on "services" at the top, then on "Buying and Selling Tools" on the resulting page, then to "End my auction early" under the main heading "Manage Your Items for Sale."