I've seen some say for authentication but staffing employees able to perform that authentication at even a fraction of existing GameStop stores would be extremely expensive (unless it's terrible authentication) so it again devolves to basically being a UPS store but for eBay shipping items out to be authenticated. Similar to what GameStop does with Pokemon cards and PSA grading except you can't really slab a Rolex or handbag so the authentication is only good so long as the good remains in the hands of the authenticator.
The only small service I think they could offer is a way for sellers to certify what's being shipped to avoid contests, but having that in every GameStop store is also very expensive for a cost they currently just shift onto sellers by siding with the customer. eBay could easily implement that if they wanted by partnering with UPS stores or a program where sellers video the packing or something.