Halacha is flexible when it comes to some monetary laws. Sometimes the financial norm (minhag) overrides halacha. For example: By the letter of the (Jewish) law, marking down a customer’s name on a product doesn’t mean that it’s his yet. However, in certain communities it was accepted for customers to jot down their names on products for sale in order to “mark their territory” and show serious interest. If business people in that community view that action as full kinyan (full purchase, a point of no return) then so it may be, even though this kinyan is not written anywhere in the Torah. The same applies to handshakes, or any other action which human beings decided means business.