First go see the machine, try it out, how does it feel?
Try to find out what she paid for the machine, you can ask to see the initial invoice or do research to find the answer. How does this compare to what she is asking. You are just looking at the base machine at this point. Keep in mind the original purchase probably included set up and delivery. When buying second had, generally the buyer is responsible for this.
Remember a car depreciates by 25-30% the moment it leaves the lot. Then it continues to depreciate.
I have no idea on the pricing for Innovas, so my comments are entirely based in general on buying used items and how I do it.
I would offer $11,250 or 75% of her asking price all in. This would include all the accessories she listed. You can work your way up from there.
The extras may be worth a couple hundred dollars, or more, but without the machine are not really worth anything. It can be a challenge to sell off individual accessories, that are machine specific.
Now having said that the machine has been for sale for 4 months. It could be that either she is not willing to budge on price, or is getting fed up and wants it gone.
There is a Facebook group for selling LA's that you may want to check out, it will give you an idea of prices.
https://www.facebook.com/groups/Longarms4sale/