Is it permissible to engage in a Murabaha transaction with an Islamic bank to purchase building materials, in the event that some of the materials are not available with the merchant the bank deals with, and the merchant obtains them from another merchant, or gives me their value to purchase them from another place?

This transaction is a forbidden usurious loan because the bank pays the price to the merchant and collects it from you with an increase. As for a sound murabaha, it is when the bank purchases the commodity, and it enters into its possession and responsibility, then it sells it to you at a profit. It should not be sold back to the first merchant from whom the bank bought it, but rather to other merchants.