In Persian culture, there is a principle named taarof in which if something is being offered to you, you must decline it, even if you want it. The giver will ask again if you would like the object but even still you must decline it. This process repeats one more time. Then, if the giver was offering out of politeness but didn't actually want to give it to you, he would no longer ask if you would like it. However, if on the 4th round, the giver asks if you would like the object, only then can you accept this object. Here is the Wikipedia page on it: https://en.wikipedia.org/wiki/Taarof
Let's say that you know about this custom. You go into an Iranian jewelry store and ask for the value of a necklace. As per taarof, the shop owner says it is worthless ("ghaabel nadaareh"). You know this is taarof but you thank the shop owner and take the item free of charge. You are aware that her offer was mere etiquette but you took advantage of her and 'stole' a necklace.
Is this halachically allowed? If not, is this considered a robbery in public or robbery in private? What penalties must he pay?
P.S. I am aware that this is extremely unethical and goes against all ethical and moral values. This is a hypothetical question. I am not actually going to do this.