Because the couple likely brought a big wad of cash in order to buy a car. I don't know why they couldn't have just robbed the couple without killing them... senseless.
Because they saw his face.
One relatively safe way to do a cash transaction is to have the guy meet you at your bank. Go inside, where he knows he will be visible on bank security cameras, take out the money, have him hand over the property in front of witnesses and bank cameras, hand the money over, it's done. You might even be able to have the bill of sale notarized there.
And as others have noted, have a good friend in the parking lot as armed backup and overwatch.