I'm a former realtor and life long NYC area resident. Outside of the NYC area, it's much easier to sell by owner and be successful than it is in NYC.
Some facts, truths and what realtors won't tell you:
1. In NYC, agents are ruthless, greedy and manipulate everything to make deals work for them. Fortunately, it gets a little better in the NYC suburbs but I was warned that NYC is the wild, wild west compared to the suburbs.
2. If you are not listed in REBNY (the MLS system), expect agents to ignore you and they will show their buyers every home except yours. On the flip side, agents do scan Zillow, Trulia, etc. to contact For Sale By Owner listings. They call you to tell you that you're stupid and don't know what your doing hoping to convince you that you need to list with a broker.
3. NYC agents do lie. If buyers ask about your home for sale because they saw it on Zillow, Trulia, etc., agents will tell buyers that your home is not for sale and your listing on Zillow, Trulia, etc. is old and your home has been sold already.
4. Safety. Since it's NYC, how will you screen buyers before showing your home to them. When selling by owner, you need to have buyers email you either a pre approval from a lender or proof of funds. You need to verify the information before showing your home. Unfortunately, there are people pretending to be buyers and have criminal intentions instead.
5. You need to remove all personal, financial and sentimental items from your home and put it in storage or at somebody else's house. This includes, all bank accounts, any paperwork with ID numbers on it, jewelry, etc. If you plan to hold an Open House, you don't want anything stolen or used for ID theft.
6. You need to know that Brooklyn is as expensive as Manhattan now and Queens is catching up (getting pricey). Buyers have been looking in the suburbs for awhile now because housing is cheaper, the public schools are excellent and transportation to Manhattan is 30 min. from most places.
7. Finally, Google "Flat Fee Brokers". This is an alternative to full broker listings.