How to find listing broker's identity?

I am interested in an apartment. The listing I found does not have the address of the building, although I just independently obtained that info. I just asked the broker who had the apartment listed on her company's web site if she would co-broke with me. She said the listing is just "not a co-broke offering" and she therefore can't do co-broke, but she will show me the apartment as my "buyer's broker." What's this about? Only thing I can figure is that someone else has the exclusive listing as the seller's broker, and the broker I've been chatting with is trying to reel in buyer's as her own clients. That said, How do I found out who is the seller's broker? A friend has access to REBNY but we just don't know how to find this out. HELP! Thanks!