Broker payment ...how much?

We had a broker send us an unsolicited letter informing us he had a buyer he represented who wanted to purchase our apartment. Our apt is not for sale but quite simply im open if there is an offer i cant refuse. They have not seen the place but there has been several units in the same line which sold recently. Is the broker likely to make me sign something before she brings buyer by to look. Can i have them over, refuse to sign anything, and just structure the contract thereafter. If i find a 3pct commision too high can i just tell them ahead of time..im only willing to give you a 2pct comission on this transaction?