Thats a hard one, ebay is your best bet really, as bikes on there seem to go for pretty top dollar if they're sold well. Take a look and see what the going rates are.
The thing I always think, is that I can normally get pretty much any bike for like 20% off msrp in New Zealand if you wait for a sale (many bike/outdoor stores in NZ do 20% off everything sales once or twice a year), so I would think for me a one year old bike in pretty sharp condition would have to be at least 40% off retail, otherwise its just not worth the gamble.
eBay seems to be changing this though, as people seem dead keen on paying retail for something through some person they don't know, rather than going down to their local store and buying with warranty etc.
What it comes down to, is that its worth whatever the most someone is willing to pay for it is.
A used bike could have been treated like gold or abused by the original owner. When buying used you never know what you are going to get. Check your local bike shops for deals on Stumpys. I just picked up a 06 SJ for $1600.
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