While this is ultimately your business decision, you could tell the buyer that eBay is collecting the sales tax and you have no control of when it is charged or how much is charged, so they will need to contact eBay customer support.
Or, if you are charging sales tax in a state without a marketplace facilitator law, you could tell the buyer that the state requires that you charge sales tax and that you are in compliance with state law.
Sometimes your buyer might complain about being charged the wrong amount of sales tax. Unfortunately, as explained above, eBay makes it difficult to charge the correct amount of sales tax to every buyer because they only allow sellers to charge one rate per state, and many states have multiple – even thousands – of tax rates.
In the rare case that a buyer notices they were overcharged sales tax, you can explain eBay’s limited system or issue a refund.