Planning to buy used Alienware 17

Hello Friends,

   I am planning to buy used Alienware 17 (i think ts R4). I personally don't know the seller. I found ad. online. Its 8 months old with bill and warranty. He said he hardly used it and in mint condition (But we can't trust unknown people :( ). He is selling it for Half the price.

How do I know if the condition of laptop is real good and the parts are original and working fine?

I cannot open the parts of laptop, as its not mine :D Please help me from getting cheated. 

How do i know the warranty period? Can I find warranty period online?

What are the think I should keep in mind while purchasing used Alienware.

Instead of saying, don't go it :P.  Please help me with this guys.

Thanks in advance :)