Is what I am doing – assembling computers and selling them to the company at the market price of the assembled device, even though their cost is lower – considered a form of deception?

If the agreement between you and the company is to procure devices for them on an agency basis for a fee, then you must purchase ready-made devices for them according to the agreed-upon specifications. You are not permitted to take anything other than the agreed-upon commission, as this is the requirement of trustworthiness in agency.