Does a sin offering (kaffarat yameen) become binding upon me after I swore to a customer that he would not take or buy anything from the shop, and then one of the workers sold him what he wanted?

An oath is based on the intention of the swearer. If your intention was not to directly sell to the person mentioned, then you do not break your oath by someone else selling to him. However, if your intention was to prevent the person from buying from the shop at all, then you have broken your oath and an expiation for breaking an oath is incumbent upon you.