Quick Off Market Townhome Sale in Park Slope

I have a three family 2,700 sf investment townhome in Park Slope (around 6th and 6th st). I have a need for some cash quickly to potentially pursue another investment. I was wondering if it was possible to sell this to an all cash buyer in 30 days. Has anyone had any experience with this. I have received those mail flyers from the Dolphin Equities guys but would prefer to sell to a user. Could anyone recommend a broker who works with all cash buyers.