What is the ruling on installing surveillance systems for homes and commercial shops, while purchasing the goods based on the customer's recommendation, and setting a profit margin on each item and for installation fees, then paying the merchant the price after the completion of the work for the customer?

It is permissible to agree with the customer to install the surveillance system and supply the devices. This falls under the Istisna' (manufacturing) contract, which is an exception to the rule of selling what you do not possess, provided that the specifications and type of the devices are clearly stated, and the deadline and price are determined. If the customer does not desire the bundled deal, it is possible to deal with them through an agency agreement for a fee, or a Murabaha (cost-plus financing) sale, or a Salam (forward payment) contract.