My standard line on used goods is around 50% (whether I'm buying or selling), and I don't want to hear about how it's only been used once by your grandma on a corduroy day, I don't care. But that's just me.
BTW, a quick UK-based Internet search shows this ski, new, mounted, at around GBP400, or $600. If you can find it at that price in the UK, I'm sure it would cost less in the US. Maybe it's a different ski or something, but certainly looks the same to me.
Also, while it's true the bottom line is "what the market says," as a buyer, I say:
-Discount for not knowing the seller
-Discount for not being able to verify the claims (how many times used, the thing about the tune, which I don't really get, etc.)
-Discount if ski's not the exact length I want, binding not the exact model I want, sole length not exactly the same as mine
-Discount for ski being used, even once, even for one turn. New is new. Not new is used
-Discount for risk/hassle of payment via Internet to a private seller, arranging for shipping, etc.
My bottom line: when it comes to used gear, the buyer is in the driver's seat.