So the broker you're working with and you have signed off on the NYS Agency Agreement form yes? This is required by NYS law and has to happen at first substantial contact. Based on your statement of working with a broker, I believe you have a buyers broker agreement. You can work under that agreement but YOU can pay your broker's commission yourself. It does not always follow that the seller pays and that's a fact. Consider that and if it works for you, go for it. The listing agent won't care as it won't affect their money. Of coure if you don't want to do that you won't be in any other position than you are now. By the way, if your broker did make you aware of this listing, they would be considered the procuring cause of any resultant sale and can sue you for their commission if you move forward without them and they have some documentation of advising you on this property. Good luck.
Licensed Real Estate Salesperson
Sure you can complain to the LA's broker but last time I looked, there was more than one apt for any client I ever showed rentals to. And by the time you gain access, its likely to be rented anyway.
Good luck. By the way, I would also suggest finding another broker to hang your license with. If you can't talk to him about something as basic as this, you need to be some place else.