An AI-Agent browsing eBay for a "widget" for a given individual will also likely not be browsing eBay's advertising listings (sponsored and promoted listings [1]) which would potentially equate to a loss in ad revenue for eBay. So there is likely a "protect the advertising moat" aspect to their "ban" as well.

[1] Given how hard eBay pushes sellers to purchase the sponsored and promoted listing tiers (at an additional fee of course) implies they make some nice revenue stream from these advertisements.