Regarding new sites like quikr and olx and other sites

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These sites were used for us to buy and sell but i dont know how they earn a lot because i even sold some computer spares through quikr but i did not paid any money or commission to that site since after i posted my add i received calls and directly sold to people so i know that they earn through ads from other sites and products for publishing in their site but do they make so much money if so many would have started a site for buying and selling anythings