I thought this was a very interesting thread. So often I see well overpriced second-hand knives and regularly make offers which, after the seller has had the listing running for months and months, they end up going even lower than.
I'd agree mostly with Peter, but have a few other considerations.
If you can try to price them in the market. As stated already it depends on demand for the knife. If it is something special you might get close to retail or even more than. I would say 10-20% off an almost new condition. Nobody is going to chance dealing with you at the same price they can get it new from a dealer. If they show wear, they are much harder to sell, especially in this market. Most people want non sharpened and non scratched or they pay user prices which can be 25%-40% off easily.
That is at least how I see it and is based totally on what I follow which isn't many brands
Of course if we are talking about an out of production, or custom knife, well pricing is whatever the market will bear; this is not the area I'm really looking at for discussing price.
Some out-of-production knives increase in value, and others don't, so not being a current model is no guarantee of holding its price.
Let's take a current production model and work it step by step:
When I value a knife like this, I start with the 'best new price' I can find including any forum or other discounts. This is often less than you paid for it.
Take this best new price and immediately drop at least 10-15% (all in, fees and postage) - this is the minimum factor by which a buyer is prepared to risk a private purchase over a dealer purchase (which is guaranteed), and requires a perfect, never used, boxed knife.
Then we have to look at condition...and condition is very subjective.
-has it been sharpened (how well has this been done)?
-has it been scratched (due to bad sharpening or something else)?
-is anything missing (box, strap etc)?
-has it been carried (even if not used)?
Any of the above will start chipping away at the value, and even with only light use, these factors should quickly get to 40% off the best price (as Peter said) or more. and for heavily used, I'd expect it to be priced at less than 50% of the best new price.
The OP's description of 'still in pretty good shape' rings alarm bells for me, sounds heavily used, and would have me thinking in the region of 70% off best-new-price. Clear photographs should explain better though.
Think of it like a car; it immediately drops value the second it leaves the forecourt. Then higher mileage, scratched paint, dents all have formulaic reductions and dealers use these to get to a value.
Be realistic, honest, fair and use plenty of clear photos - the buyer must know what they are getting.
