Find the answer to your question
How can I cancel bids on my items before calling EndItem?
The cancelling of bids is not an API supported feature. All auction items which have bids will end with the high bidder as the winning bidder when an item is ended. Therefore you need to be very careful when using the EndItem API call. If your intention is to remove the listing entirely, you should first cancel all bids on the item, and then end the item.
You can use the following URLs to cancel bids on an item you listed.
Please note that the link to the cancel bids function is linked to from the Site Map, with a link title of "Cancel Bids on Your Listing".