No way is a used cable worth only a $30 discount. If it's actually brand new (they never used it) and you can access the warranty, I might go as high as a $50 discount, but otherwise no higher than a $100 discount in my book.
P.S. My general rule for used electronics is start at 50% of what it sells for new today, not originally, then add or subtract depnding on condition and availability. If it's like new or hard to find, then offer more, and if they are plentiful, or the condition is not great, offer less.