I have seen chopweilers sell from 300-400 in the last year or 2, New and used, with and without sheaths.
I got mine for 300 in mint condition with NO sheath.
For yours with sheath, since its used, id say 350 is a decent "fair" price. But id likely start it at 400 and see if it had any bites before reducing it slowly down to 350. I wouldnt take less than 350 personally and feel its worth well over 500.... But this is just my opinion. Since its a knife that is no longer made you can ask what ever you want for it.
But again i think this is one of the best knives to leave the swamp, so id never sell it. Its not easy to get back once you regret selling it. But im sure someone will buy it up with no problems.