Other than making sure you buy a snowboard that is right for your height, weight, feet size, riding style and ability, there are few other things to look out for when searching for second-hand snowboards.
Check the top/deck for any visible damage. Delamination of the top-sheet can often be repaired but damage to the exposed fibreglass or even to the core of the board is not as easily rectified.
Make sure you remove the bindings from the board before you buy as they can sometimes be hiding cracks or other damage. Likewise, always ask that stickers are removed for the same reason. Inspect the holes where the bindings attach and if you can, test the screw fittings to make sure they have not been threaded or damaged in another way. There is nothing worse than getting a board home to find that your bindings cannot be attached due to damaged screw fittings.
The metal edges running around the outside of the boards should be in good condition. Any rounding or burring of the edges is usually easy to fix in any good shop but deep gouges are best avoided. Likewise, edges that are very thin suggest the board has been serviced many times and it may be nearing the end of its life.
When inspecting the base of the board, be wary of cracks or deep cuts as these are difficult to repair. Scratches and scrapes are a certainty with a used board but they’re usually just cosmetic and won’t affect the ride. Depending on the style of board, it should also have a good camber and be responsive when pressure is applied. Place the board on a flat surface and you should find that the nose and tail make contact, whilst the middle of the board should have some ground clearance. Some boards are designed to be stiff or have minimal camber, but unresponsive or flat boards can often suggest the core has lost a lot of its life.
Snowboards will naturally pick up imperfections here and there but most are superficial and can be restored. More serious damage however should be considered carefully before buying as the ride may be inhibited and potentially dangerous.