If you use a used BCM that the numbers match up with yours in a Pontiac Aztec,whats the difference ?

I have to replace my BCM (body control module) its highly expensive,if you can find a good used one at a junk yard or salvage yard,and the numbers match up.what is the difference between using it and buying and using the new one ? The new ones have to be reprogrammed,but the used ones would already be programmed for the same type of vehicle.Is it possible to use the used one.What are the risk ?