All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If you agree with your friend to offer his goods and promote them and you add an increase on the price that he sets, so that the increase will be for you, then there is nothing wrong.
Ibn ‘Abbaas issued a Fatwa that this is permissible.
It is not known that there was any Companion who contradicted Ibn ‘Abbaas on this as stated by Ibn Hazm and Ibn Qudaamah and others.
Imam Ahmad said the same thing as well.
Based on that, there is nothing wrong with that transaction if you agree with your friend on the increase.
However, if your friend asks you to post his goods on your page and sets a price for you, then you increase that price without his knowledge so that you will take the difference for yourself, then you are not entitled to do that, and the extra amount is for him unless he is willing to leave it for you.
Allah knows best.