Often times Craigslist is your best option for selling your old or unused surfboards quickly and effectively. People pay you cash and you can often sell a surfboard within a couple days if you price it right.
The flip side is that you can often find really good deals if you are looking to buy a surfboard on craigslist. Obviously the brand and condition of the board add the most value and if you know what to look for then you can often save alot of money.
Right now is the ideal time to find some good deals on Craigslist as summer is over and many people are beginning to clean out the garage and sell some stuff. Not to mention at the beginning and end of each month alot of people are forced to sell their boards to get rent money. By being patient, knowing the prices of surfboards and finding the right seller you can often score a great board at an even better price.