I would definitely keep it super cool, document all of your interaction (someone videotaping) and go about getting in touch with the lady who sold you back your snake. If she is co-operating, then that's your best bet. I would call the police and let them know what's going on, and ask them their advice on what to do in this situation. You don't need to press charges, but they might have some advice for how to handle this in the most effective way. Plus, if you ever did need to press charges, you would have the backing of the local law enforcement and showed that you followed the proper steps before it was an issue.
Just my two cents.
Then again, I feel that she severely crossed a line with selling your pet.