What is the ruling on profiting from buying and selling computer spare parts to companies or friends with the intention of trading? And what is the expiation for it if it is unlawful?

If purchasing parts is part of your contracted work, then it is not permissible to increase their price. However, if the company offers you to buy the parts so that you may then sell them back to them, then it is permissible to sell them for whatever price you wish. This ruling also applies to your selling to friends. If you have taken an undue increase, then you must return the money to its owner.