A rule of thumb I have used for buying anything from the internet is 30% for new items. This applies to camera gear, scuba gear, computers, and now hiking gear. If I can't get 30% below the local dealers prices, I will buy local and watch for their sales, hopefully 20-25% off. I would think used should start at 50% off for something that is basically in new condition and go down from there.
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Just because you don't take an interest in politics doesn't mean politics won't take an interest in you.... Pericles (430 B.C)