1099-K is Activity; it's not really income or expense, but Financial transactions from the settlement agent (Amazon) to your client.

Perspective matters for sales taxes. Sales tax collected for sales from customers for the selling entity for sales made is Liability; not income or expense. Sales taxes Paid because of your own purchases are an expense. Sales taxes taken from settlement funds on your behalf, when you are the seller and Amazon is the settlement agent, is not your expense or income because it was collected from those customers and sent on to the taxing agency.