Questions about brokerage agreement

My client (I'm an attorney) has spoken with a major broker in NYC about selling his coop apartment in Brooklyn. I am reviewing the brokerage agreement they gave him. 1. How common is an exclusive right to sell in NYC? Is this what he's going to get from all the major brokers? Will the "majors" give him an exclusive agency if he asks for it? 2. This contract does not include a term or end date. Why not? 3. What is considered "willful default" by the seller? 4. What chance do I have of adding language giving the seller a right to terminate? Of course any contract can be negotiated, but I'm trying to be practical. We're talking about a roughly $300,000 unit by the way. Thanks for any input on this!