What is the ruling on taking a percentage from the seller of machinery without the buyer's knowledge? And what is the ruling on the increase added by the broker to the price of the machinery without an agreement on commission with the manufacturer?

What you are doing, taking a commission for brokerage, is permissible. Brokerage is considered a form of ju'ala (commission/reward), and it is a condition that the payer of the commission must be aware of it. You must be careful not to promote goods falsely. Your friend selling a commodity before possessing it is not permissible, due to the Prophet (peace and blessings be upon him)'s prohibition of selling what you do not possess. The legitimate alternative is for him to act as a broker for the owner of the commodity for a known fee, or by taking what exceeds the specified price.