What are the trade-in requirements?

We have bought 3 cars that have died.  I'm looking for a financed car,
hopefully with an extended warranty.  I'm helping my son and don't have a lot
of money, BUT we can contribute 2 cars with his car that just died.  
QUESTIONS:  (1).  Are cars accepted as TRADE-INs regardless of whether
the cars run? (2).  If so, HOW MANY CARS can you trade-in towards a down
payment. (3).  Does CarGurus buy cars regardless of if they run?  THANKS!