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 Edit Listings, then Best Offer.
In the popup, select to Edit this page of listings only or Edit all listings at once and select Next.
If you selected to edit this page of listings only, on the next page you're able to enable or disable Best Offer per listing or for all listings at once. You may also set the auto accept and auto decline (optional) per listing or for all listings at once.
Once you've completed the changes, select Save all modified.
If you've chosen to Edit all listings at once, set your values in the popup and select Edit Listings.