What should I do with a wedding dress whose owner left it with me to sell for her, and the shop has since closed and I have been unable to contact her for more than three years?

The garment or its value must be returned to its owner if her whereabouts can be identified. If a year passes from the beginning of the search for her and her location remains unknown, it is permissible to use it or sell it and donate its value as charity. If her owner is found after that, she should be informed of the situation. If she approves the charity, she will receive the reward for it. Otherwise, the value should be returned to her, and you will receive the reward for the charity. This is because any property whose owner is unknown, such as borrowed items or deposits, falls under the ruling of lost and found items (luqata).