Advice from those who have sold cars either now or in the past

I have a friend (no, not me) who is looking into selling cars. We talked about it. I only know my experience of buying cars.

If you have sold either new or used cars now or in the past, what bits of wisdom would you share with this guy? He's on the fence. They are promising some decent money (especially for my friend who's never earned 6 figures in his life).