In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Umar Ibnul Khattaab [radiyallaahu-anhu] said: None is to conduct business transaction in our market places, except the one who has acquired understanding in the religion. [Ref 1]
Shaikh Ahmad Shaakir [rahimahullaah] said: Yes, until he knows what to take [from the people] and what to leave; until he knows what is Halaal and what is Haram, so that he neither corrupts the buying nor the selling of the people with falsehood and lies, nor enters them into usury from any angle whilst the buyer is unaware of that. Trade should be based on sound Islamic principles, so that both a Muslim and Non- Muslim are satisfied with it- free from deception and swindle. [Ref 2]
[Ref 1: Sunan at-Tirmidhee’ Number 487. Declared Hasan by Imaam Albaani (rahimahullaa)]
[Ref 2: Tahqeeq of Sunan at-Tirmidhee 2/357]