I was surfing eBay and I came across some Offical Xfinity Gateways for sale. Can I activate the Gateway without rental fees if I buy it from a third party retailer?
Comcast takes the position that, if a device is not listed among the Retail devices on https://mydeviceinfo.xfinity.com/ and does not appear on http://mydeviceinfo.g.comcast.net/ with a check mark in the "Retail" column, it is not legit for retail purchase.
They may not activate such a device. And even if they do, they are likely to charge the rental fee for it.
Adding to BruceW's post;
FWIW, one should be careful if buying used modems on sites like ebay. Many times they wind up being classified in Comcast's database as having been stolen / unreturned and CC won't provision them for service. To be safe, buy only brand new modems in their original packaging from reputable retailers.