If eBay has approved your account for Strikethrough Pricing,
visit the Listings Manager's Settings
and check the Enable MSRP features option:
After saving, the Listings Manager will now include:
a Retail Price column to sort and edit by Original Retail Price
a new Quick Edit bulk action to edit Original Retail Price
a new input in the listing editor to edit Original Retail Price
Use Report Center --> Custom Report --> Report Type: "Listings from eBay" to export OriginalRetailPrice.
You can then make changes and re-upload the CSV to update Retail Price in bulk.