As I stated and you quoted, unless the price is already reasonable or the item is actually collectable I'd offer 50% of the asking price. In your specific example I wouldn't offer you a dime, I don't knowingly buy knives made by people like Mick Strider.
We or at least I'm talking about buying used knives, not something that normally appreciates in value. I don't buy "collectable" or "art" knives. I'd never offer 50% of the sellers asking price on a collectable or a rare knife because I have no plans to ever buy one.
Not trying to start an argument, I'm just amused that people would be insulted by someone making an offer on a used item that the seller wants to sell. If my offer, or any offer for your used knife is too low, turn it down and take a better one. Seems simple enough.
I used Strider as an example, whether or not you would buy one of his knives is a whole other debate.
So if I had a Spyderco PM2 that lightly used, and listed it for $100 and you were interested in it, you would offer me $50 ? I'm just trying to fully understand you here.