To change fixed priced listings to auctions in bulk, in the library, make your listing selections and click the Bulk Actions button.
The Bulk Actions button will appear towards the top right of your screen once at least one listing is selected.
Choose Listing Actions, and then Fixed to Auction.
On the next page, select your duration and enter your pricing per listing. Then select save all modified.
If you're using linked listing profiles, the listing type may not be bulk edited from this page for those listings linked to the listing profile. To edit listings using the profile, please edit the profile or unlink the profile from the listings by bulk editing in the library and assign the listing profile is 'none' option. Then continue with the listing type bulk edit.