Personally, I put a value on used stuff (not just boarding stuff, all Craigslist type things) at 50% of new and work down from there. Unopened box 50%; opened, but only used once or twice 40%. I start there and tweak it up or down depending on the item, it's rarity, durablility, etc.
Sorry, but something as common as a used Burton snowboard setup that I can get for $400 new with a warranty? $200 tops, maybe a little more if I don't have to pay shipping, less if I do.