I'm thinking 60-70% is a low asking price. If it's in superb condition, put it at 80% or so of the original price so you have room for negotiation. If you get no takers, you can always come down a little.
I hate that secret squirrel room for negotiation crap, put the price you want for it. I wouldnt ever pay 80 % of new for a used item from an unknown seller. When I can wait for a sale and get it from a store that has a return policy and a business acount to ship alot cheaper.