DS items are frequently on eBay, still sealed in bag. I'm not sure how they could stop people doing it, since I doubt eBay would release their details.
If a Geobra employee bought one item from a seller they wanted to investigate, they would then gain access to that seller's contact details on eBay, which might well tally with details on their own files (Paypal email address, DS order email address, for example). No need for any special agreement with eBay. I am not advocating this, you understand, but there are simple of finding out who is selling DS items if they want to! There are also ways of avoiding detection too of course, like using different contact details for eBay and PM.