I'd rather have angry NPCs coming after me than other players.
I just sold a car in a "solo" session without lowering it's value. Here's how i did it. Might help out to someone who's doing the same thing.
Now I'm using the cheapest warehouse located in La Mesa.
NPCs spawn and come after you with 600% accuracy roughly 100m from your warehouse.

So here are the steps:
- When you start a selling mission follow the red line.
- NPCs usually spawn at the red circles so when they do, park and exit the vehicle.
- Ways:
- Wait for them to get out of their cars and just shoot them. It's easy but effective.
- Limo (CEO Menu), get in the back and fend off waves of NPC cars.
- Rhino. Obviously. If you have it.
- Buzzard (CEO Menu), buttbeware, they take cover near your car. So be careful not to blow it up.
- Kuruma. Because why not. - You get attacked 3-4 times, 6-8 cars. Just wait it out.
Then get in the car and safely drive it to the destination, preferably without crashing to get the full amount.