Di,
I am glad you took your time with the machine and asked questions. I think your offer was more than fair, especially considering the seller was not upfront in her ad regarding the rental use and being a business owner.
Have you called Innova to ask what may be causing the vibration? I would ask that question before going any further.
I would also call them to ask how to check the stitch count. It seems somewhat unbelievable that they cannot determine it on a 5 year old machine. My HQ is 15 years old and it is very easy to check the stitch count.
Now I do not put too much weight on stitch count, these machines are designed to have millions upon millions of stitches. I would be more concerned about buying a rental machine.
If she has been using it as a rental for the past 3 years, she has been both making money off the machine and having a business write off. Her asking for basically what she paid for it is pushing her luck. I am glad you offered about 75% of her asking price.