Because agentic AI can parse unstructured data and make purchasing decisions regardless of whether your site allows it, which avoids the chicken-and-egg problem.
It's similar to DoorDash. If your restaurant didn't want to sign up, they added you anyways and took orders on your behalf, then sent a physical courier over with a prepaid card to order takeout. Sometimes the menus were parsed incorrectly and customers blamed the restaurant.
This forced restaurants to sign up, claim their page, and keep their menus up to date, since not offering delivery wasn't an option.
At least 1 agentic AI tool will ignore these new terms and buy stuff on eBay anyways. Inevitably there'll be bugs or it won't get the best deal. At first this won't matter, but eventually competitors will offer a bug-free purchasing experience and consumers will move over.