frankly, alot of residential rentals are not listed on MLS because brokers don't think it's worth their time for the money, and owners don't think it's worth their money for the time.
most of the rental listings on mls are probably favours for investors who bought a condo from the agent/broker in the first place.
additionally, some of the older apt buildings are owned by REITS who have their own marketing .... again, not on mls
I believe there's a reason why owners don't think it's worth their money. It's super easy to rent residential units with all the available websites to advertise your unit and the demand. If these unreported numbers were included, I would think it'd drop the vacancy rate.
As long as you are able to get the following with no exception, your pretty much covered even without an agent.
-Employment letter or school acceptance letter.
-previous landlord reference
-First and last month rent
-Credit check (I don't always do this)
-A lease (If you're not looking for short term)
Just my thoughts. You may feel more comfortable using an agent.