On the average, I don't even find that flipping the odd machine even pays for itself or frees up any cash. Typically something else comes up around the same time that I want and uses up all of what I "made" on the machine I flipped which means I just paid for the parts on the machine I flipped out of my pocket.
The other thing I noticed no one has mentioned is that rarely is a flip sale a fast one. You either have to show them how to use the machine - an hour with questions - or a couple of hours while they decide on a machine then want you to teach them to sew.
The only way I've made anything is selling parts and quilting accessories and servicing other people's machines.
OH! And speaking and teaching.