It should be clear that celebrating Black Friday or other religious occasions of non Muslims is not permissible because it creates similarity with non believers. In addition, it is not permissible to perform any act that endorses such events or adds luminosity to their events. If a person celebrates these events to show respect then it is considered kufr. However, there is a possibility of buying and selling on this occasion without any intention of showing respect.
Allah Knows Best
Darulifta Al-Ikhlas, Karachi