There aren't many places for used boards. Surf Garage or Used Surfboards Hawaii. Might be a rare beast but you never know. Price will be reflected with the condition of the board.
Try facebook marketplace.
Place a WTB ad also.
Nice longboards go really quick! There was a few Thomas and Dead Kooks on occasion. Myers, Craig Kawamura, Vesso and Manuela Shapes from Maui are some nice designs.