ANSWER:

If you are acting as his agent, you must first agree with him to the commission he is going to give you. You can’t sell it and make a profit out of it without his knowledge as this would be betraying his trust.

If you want to make a profit, you must tell him that you will sell it with a price of your choosing and keep the profit. 

If someone gives my his shirt to sell either as an individual or in my business.is it permisible. And another thing is that if he gives me a specific amount to sell it and for me as am selling i add some amount, i give him his amount he said i should sell it,is this riba. And after selling him not knowing he gives me some appreciation fee should i take it.