If my 17 year old took my car and had an accident but there was no insurance because the car was being sold, what to do if I am being held responsible?

You are potentially doubly responsible: first, as the car owner; and second, as the parent of the minor who had the accident. If your child is at fault in the accident (e.g. was driving carelessly), you could be responsible for any damages, injuries, losses, etc. he/she caused.

Without insurance, you'd have to pay any judgment personally; if you do not, potentially your income could be garnished, money could be taken from your bank account, a lien could be put on your home, and/or some of your property could be seized and sold. You may need to consider bankruptcy as an option, if the amount of any judgment is unsupportable for you.