Thanks. Another couple of random things;
1. I don't think enemies spawn when selling a car if there's someone else in the session with you who isn't in your organisation. I was in a solo public session when Jizzy joined, when I went to sell the car there were no enemies. However the downside of this is that once one person joins your session it seems to open the floodgates for others to join.
2. For certain steal car missions where a wanted level is involved you can enter the military base without turning it hostile until you attack someone.
3. There are 32 vehicles to steal, once you get to that you'll start getting the same ones repeat. So if you want to keep selling a car over and over again get your garage up to 32 then sell the car you want, whenever you steal the next car it will always be that type - I always sell the T20 because it handles well... there are three variants; TOPSPEED which is yellow, DEVIL which is red and CARAMEL which is a shitty brown.