Completely against the "rolling coal" idiots, but how is eBay on the hook for what sellers put on their platform if they already remove 99% of it (as claimed) through automation and manual means?

They aren't the seller, but the marketplace. Its basically section 280. They've defended against these kinds of cases before.

Go after the sellers or manufacturers, or put in place a system with fewer loopholes. For instance, in my home state of Maine, emissions aren't even considered, and even a check engine light and gas cap check, the most basic verification, is only required in one county. Elsewhere, their eight-inch lifted truck with off road light bars, tires sticking out ten inches past the wheel arches and a single eight inch exhaust in the flatbed breeze right through the yearly "inspection."

Plus, the cops don't care, and likely do it themselves on their drunk way home to beat the wife.