Marketplaces like eBay are designed to monetize 'Wandering Attention'—sponsored listings, 'customers also bought', and sidebar ads.
An AI Agent represents 'Laser Focused Attention'. It executes a transaction with zero wandering. It effectively turns the marketplace into a commoditized backend database / dumb pipe.
From a Growth/Unit Economics perspective, an AI Agent is a nightmare customer. It has zero probability of impulse buying and generates zero ad revenue. They are banning them to save their business model, not their inventory.