when you say part time, do you mean you are continuing the mechanic side of things or are you doing something else so you can only do the sales part time?
I think your mechanical knowledge will help you, you can buy stuff where you know how to fix it which may scare others off. As David has said, different online channels work for different people, you will find one that works best for you by trial and error. there is no answer to the length of time to sell a car, but prep well, quickly and take good pics, know your prices and you will improve speed of sale.