[Quiet Man Cometh]

Sueing her would cost a heck of a lot more than $500, so it might not be worth it. You could report her for theft if she does go through with it I imagine, but it wouldn't guarantee you got them back. At least as far as I'm guessing. I'm only really aware of the Canadian legal system and that probably won't help.

Your best option I think is to not let her sell them in the first place. If you live away from her, maybe have them shipped to you? It will cost more money, but it might be worth it for peace of mind.