Legal Question in Business Law in California

If you and she agreed on the price, she paid you the amount agreed, and you delivered the watch, the deal is complete. There is no basis for a lawsuit, unless the agreement included a contingency that if the watch were subsequently appraised at a greater amount, she would pay the difference.

For what? It's not her fault if you made a bad deal. Unless there is more to the story, I'd say you're out of luck.