Ah, I think it's one of those cursed enchantments where you get luck with it but lose the equivalent amount of luck as soon as you no longer have it. Kind of like a Zashiki-warashi or a Rabbits Foot.
What is zashiki-warashi? The first part of the name sounds African, but the second part sounds Japanese..... Either way, never heard of it. I could google it, but I find conversation eases understanding for both parties.
I was thinking the seller might have a curse that would cause her to get hit by a car, but she had the crystal that was constantly protecting her until she handed it over. But I like your idea too.