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 Item Condition.
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 change the item condition for each of the selected listings.
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.
Since eBay condition values offered depend upon the specific eBay Market and eBay sub-category, the listings will only be edited if they are using the specified values for market and category.