A cable box is a two way device just like a modem. It recieves signals to decode and it also sends info back such as pay per view purchases etc. That two way communication is critical to it's functioning.
The cable company keeps track of every box and knows where it is supposed to be and if it is someplace else it will be shut off. It may take a day or two but it will happen.
A box from another sytem will not work first of all because it is not authorized in the new system and will not be activated second because every system has a different version of the software.
Cable companies won't authorize a box you bought on Ebay because the manufaturers do not sell cable boxes on the open market and any cable box you can buy on Ebay likely belongs to some cable company somewhere and the person selling it is trafficking stolen merchandise.
Let me ask you if some one was stealing from you would help them to enjoy what they stole from you or would you tell them to pound sand?