Is it permissible for me to add a percentage for myself to the purchase price or maintenance cost of computers without the buyer's knowledge?

Brokerage is permissible in the purchase and repair of appliances, and it is considered a form of ja'alah (commission/reward). However, it is a condition that the person paying the commission is aware of it; it should not be taken from him without his knowledge, such as adding it to the price without him realizing, as this is considered treachery and consuming wealth unjustly. As for the second party who is not paying it, their knowledge of it is not a condition. If an agreement is made with the selling company or the maintenance service provider for a commission to be added to the specified price, then there is no harm in that.