You have to know that no agent work without commission. Usually, for the search of property to rent (or for the search of a tenant), an agent will take one month rental commission. After this, most often, they will propose you properties where they have additionnal retro-commissions (ie. properties that they put on rent themselves – which will give 2 months commission, and properties from other agents with whom they can get a bonus). Needless to say that if the seller is an agent or an individual not giving bonus, the chance to have this property proposed to you is reduced.
So for a more efficient search, it is a good idea to also look for flat in your own, as there is no guarantee that an agent is proposing you the best properties available, but on the contrary the most profitable for them.