Unless he notices that US residents can import goods valued up to $800 US duty free, this is unlikely to affect most eBay sellers.
Being an idiot, Trump is adding duty on materials needed by American industry.
This will raise the cost of manufacturing in the USA and I've seen reports that the US will shed some 90,000 jobs as a result.
And US made goods will rise in price.
The tariffs make it a better idea to import ready for sale items, made elsewhere, than to make them in the USA.