Worth is whatever the market will bear. It's also in the eye of the beholder. For instance I as a buyer would (and did) wait for a lower price on a used CSTM, because I intended to and do beat the crap out of the thing, so it wasn't going to stay pretty anyway. But for those who want one that still looks good, $150 is below retail and probably attractive. One possible rule of thumb is to check the retail value of the knife you want to sell, on various internet dealer (not manufacturer "sugg. retail"!) sites, and if you offer the same knife LNIB for 20% under average retail, it's a fair price. But this is a very subjective area, no hard & fast rules.
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[previously incarnated as fishface, since 10/98]
the beatings will continue until morale improves