Here is link to Ending Your Auction Early:

http://offer.ebay.com/ws/eBayISAPI.dll?EndingMyAuction

After putting in an item number from your auctions this comes up:

Ending Your Listing Early
Select one reason for ending your listing early. The reason will appear on the Ended Item page.

The item is no longer available for sale.
There was an error in the minimum bid or Reserve amount.
There was an error in the listing.
The item was lost or broken.