YES you have the right to say NO. Maybe you should ask her to help you buy another car...please..no, Pleaseeee...no! PLEAAAASE!!!! It will only cost you $70 and 30 minutes of her time! REALLY! (yes, I am into sarcasm)
Make that point, give her a hug and tell her that you appreciate what she is trying to do but the easiest turnoff for people is having someone pressure them. If she doesn't understand then ask her what she has done when she gets one of those annoying phone calls asking her to switch electric companies. She is being a THEM.
Tell her..."Sue, I love that you are so excited about your business and you know that I support you (your $70 in MK says that) but you have hurt our friendship AND your business by pressuring me so much. If you want my continued support as a customer I will buy X from you and be willing to try something new once in a while but honestly I am not very knocked out by most of the products. I will help you if you have a show but look at me AS a help, not a sell. Please step back and look at what you are doing, if I feel this way so are other people that you are dealing with. How about we sit down and talk about marketing strategies and better ways to approach people? I really want you to be successful but I don't want it affecting our friendship."
It is hard when you are in businesses like Mary Kay, Pampered Chef, Amway, ect. The expectations are high and people run the opposite dirrection because of people like your friend. The successful people in the business have been hard hitters, spending thousands of dollars of their own, approaching anyone and everyone. (been there, done that) She is learning the lesson the hard way.
A real friend will realize what you are saying and eventually thank you for it. Good luck. =)