Is it unlawful to sell merchandise to a customer of a store that rejected it, and then to sell the merchandise directly to the store's customers—even if they are not regular customers—given that I am the sole manufacturer of the merchandise?

The aforementioned store acts as an intermediary between you and the clients, bringing them to you or sending them your way. If one of them purchases the product for which they came, the store receives two-thirds of its price, and you receive one-third. This is a brokerage service, and there is no harm in the store (the broker) taking a fee for this work. However, for the broker's fee to be valid, it must be known. It is not permissible for it to be a percentage of the selling price if the price is unknown to both parties at the time of the brokerage agreement, because it would then be an unspecified ju'alah (reward for a task), and this is the view of the majority of scholars. The store is only entitled to a fee for the customers it brings to you in one way or another. As for those with whom you deal directly, the store has no right to take a fee from them, because the ju'alah is only deserved by the one who performs the agreed-upon work. Thus, there is no fee for the store for a customer to whom the store offered a price and then the customer refused, and you subsequently dealt with them directly without the store's involvement. The same applies to all transactions and deals that take place between you and customers without the store having any input.