Charging a fee on the seller for taxes paid by the BUYER?
That's Standard Practice for any payment processor.
Technically, their fee is on whole payment which they process, not on any given part.
We had a merchant credit card account, through Moneris, from 1978/79 until we retired in 2014. We paid fees to Moneris on every penny we took in, from the days of the 'click-click' machine to tap cards.
We avoided paying fees on debit cards, because we had agreed to be "beta testers" for the new technology when it was introduced to Canada (from Japan) in the mid-90s, but most businesses do.
Your fees to eBay and previously to Paypal, are a business expense and deductible when you work out how much profit you eBay sales made and report them on your income tax.
