Always a tough question to figure out a fair market value on a knife. I guess the bottom line is it's worth whatever you
think is fair.
IMHO.....if I like a knife and there is some wear I usually look for 25% so difference between what I can find it for on the web. This is not scientific, this is just sort of a gage I sometimes use.
The CS Stag Trail Master retails for $350. I would never pay that price. In fact I laugh at the MSRP. All of us forumites do. Ha Ha Ha.
OK.....I got that out of my system. Next I look at what I can find it for on the net from one of my favorite dealers. In this case I checked out Top of Texas Knives: http://www.toptexknives.com/
and I can get that same knife new there for $188. That's some significant savings.
Now since the knife you're looking at is used I usually try my 25% rule which is less another $50 or so. Personally I would pay around $140 or so depending on how much I wanted that knife. Sometimes if I'm patient I can find a deal on forums like these since a lot of times members are moving out their knives. I'm not looking to low ball anyone and I never barter on the private sales section.
Places like Ebay (auction sites) may start with good prices but I can always find better deals and I find these places particularly frustrating.
I hope that helps. I would be interested to read what the others will think. Good luck with your knife and let us know if you get it.