Investors, of course. Investors make their money by purchasing properties on the wholesale market which they renovate and sell on the retail market. The retail market is served by real estate agents and properties are listed on the Multiple Listing Service (MLS).
Generally, the wholesale market purchases are completed without a real estate agent and no commissions are paid. Additionally, the properties are purchased with cash or a hard money loan, which allows the purchase to be completed in a matter of days.