Thanks for posting your question here.
You should actually be able to use the same "Buy it Now Price" and "Start Price" options within the "Advanced Edit" tool on the Saved Listings page to decrease your prices by a percentage or amount as well.
Just enter a negative value in the “Increase by Percent” or “Increase by Amount” field (depending on which option you select) and your prices will be decreased by that percentage or amount when you complete the process and confirm your changes.
I hope this helps. If you need any more specific assistance with this, please feel free to contact our Customer Support team from the “Help” tab on our site and we will be happy to continue working with you.