Technically, I suspect it should work, but unless you are purchasing this device directly from Motorola or an authorized reseller, I suspect Comcast will not activate the device; making it an expensive paper weight.
Most of these set top boxes that you find available, online, are actually the property of some cable company and are not for re-sale. These devices were simply not returned when the lessee moved on.
If this is the case and it's actually a Comcast set top box, then Comcast has every right to reclaim their "stolen" property at YOUR financial loss.
If you really want to purchase a set top box, then I'd recommend you contact Comcast and find out what the necessary specifications are and get documented assurances that they'll activate it once connected. Then go to a local store and look for a set top box that meets all of Comcast's requirements.