Sometimes people play the role of middleman or broker and sell things which they don't possess. Like selling piece of land or stock which they don't own. Yes they can sell on someone's behalf but in Islam double agent-ship is not allowed( meaning both seller & buyer don't know what share middleman is taking). You would also have witnessed many people suffering from investment loss in online stock markets.
It was narrated that Hakeem ibn Hizaam (may Allah be pleased with him) said: I came to the Messenger of Allah Muhammad(blessings and peace of Allah be upon him) and said: A man may come to me wanting to buy something that I do not possess; should I buy it for him from the marketplace then sell it to him? He said: “Do not sell that which you do not possess.”
Narrated by at-Tirmidhi, 1232; an-Nasaa’i, 4613; Abu Dawood, 3503, Ibn Maajah, 2187, Ahmad, 14887. Classed as saheeh by al-Albaani in Irwa’ al-Ghaleel, 1292.
So we if follow the principle of hand-to-hand exchange( meaning real physical exchange) and take into possession of things into our custody, we would be saved from greater loss.
Thank you for your time. If you are confused about specific issue or want to talk about anything related to Islam, you can mention them in comments and i'll try to address them in upcoming posts.