1) No, there is no need for an active listing to finalize a sale. If that were the case, there would be no privately negotiated sales, or sales directly between friends and family members, where the property was never listed in the first place.2) If you pay the realtor a commission for any sales that originated during the term of the agreement--which includes any where the buyer saw the property during the listing, even if an offer was placed later--the broker will have no grounds to sue you: he or she will be getting everything to which he or she is entitled.