For the original poster. Same advice should be applied to DTAs for sale on the Internet. Beware purchasing them because they probably won't be accepted as legal by Comcast since they don't sell those to customers in the first place. Even if they were given to a subscriber free they are still the property of Comcast.
Just for general information from eBays website on items not allowed for sale on their site.
Satellite and Cable TV descramblers: Items that can only be used to access cable or satellite television programming without the permission of satellite or cable operators aren't allowed. Stating the item is for educational or testing purposes does not make it acceptable to list. Examples of items that aren't allowed include: