buying premod modem from ebay, what other works are still required?

Hello all,

I am newbie. From reading some posts, looks like cert and mac are required, 
1. Does premod modem come with cert and mac? 
2. Is cert ISP-specific? if my ISP is spectrum, do i have to use spectrum cert? but some modems are supported by multiple ISPs. 
3. Will ISP accept two modems with same cert? what will happen?
4. Will ISP accept two modems with same MAC? what will happen? if i spoof mac, how do i know this MAC is not being used by other subscribers?

Please educate me.