Is a Nissan altima 2015 with 70k miles for 6,500 dollars a good deal
No, if you want to keep the car for over 100k miles.
No. The price seems tempting but the transmission, which is a CVT, WILL fail around 100k miles and cost you over 5 6 grand to fix, more than you paid for the car.
Before looking at a car, look for common issues with it. The common issues with the 2013-2015 Altima is that the transmission is absolute junk.
https://www.carcomplaints.com/Nissan/Altima/2015/transmission/transmission_failure.shtml
https://www.carcomplaints.com/Nissan/Altima/2013/transmission/transmission_stopped_working.shtml
This clunker has transmission issues from 60-90k miles. You are right in the middle of it. Is it really worth it? Unless the transmission has been replaced by the dealer and an independent mechanic says that it is perfect, I would run away, the first case I would pay a lot less. I would just avoid Nissans in your price range. You could easily get a twice as old Camry and it will last double what the Altima would.