Well, it's always much cheaper to arrange everything directly with the owners. All real state agents will charge commission.
Furthermore, if you use them, you are narrowing down the possible apartments as they only rent/sell those apartments whose owners have an agreement with them.
Buy
Segundamano and start calling them very early in the morning. The best flats got rented quickly.
By the way, it's difficult to tell whether a classified ad is from an owner or an agency. If you read "no agencias" (no agencies) that means the owner explicitly request not to be contacted by them to sell/rent their flat. Thus, you can be sure you are dealing with the owner.