The problem is that, unbeknownst to them, consumers received a stuffed cookie from eBay from some random website having no awareness of anything regarding eBay

The problem is that, unbeknownst to them, consumers received a stuffed cookie from eBay from some random website having no awareness of anything regarding eBay at the time. It's only because these consumers just so happened to visit eBay on their own initiative within 30 days and performed specific activities that an affiliate payment was generated. Had these affiliates not even been involved, there would be no change in the behavior of the consumers; they would have visited eBay anyways and eBay would therefore not have had to pay out any commissions. I agree with the rest of your analysis. Someone who was incented according to affiliate activity likely did encourage this, even if they suspected or had knowledge that it wasn't above board.