Please...first, there is no such thing as "the standard 15% broker fee...." Fees are negotiable but I suppose you didn't try that. A real estate license can be asked for and shown on demand. If you don't see one, move on. If you stay, thinking you can get a great deal, then you deserve to get shortchanged. Nobody lies about move-in dates especially if the apartment is currently rented. Owners can say one thing one day something else the next. The current renters can stay until the last day of the month and they all do. A landlord may want to clean, fix or re-paint prior to anyone else taking posession of a rental apt. This is where timing issues occur. Owners don't usually tell brokers because they want a renter on deck. Read Ryan's comments. I will add one thing...there are NOT lots of apartments out there. Once you add your criteria for location, bedrooms, sunlight, amenities, schools, transportation, price, etc. the availablity will dwindle.
If you're looking for excuses so you can snake out of paying a brokers fee, why don't you find something by yourself? Oh, yea...you can't.