Ask a collector what their most valuable knife is and you will get a number. Ask what they would never sell and you get something more interesting, which is what happened when r/knives ran a thread asking members to post their not-for-sale and never-for-sale pieces.
The pattern in the answers is that money has very little to do with it. The knives people refuse to part with tend to fall into a few categories: the first good knife they bought, a knife given by a parent or a spouse, a custom that took years to commission, and a discontinued model they spent a long time hunting. In almost every case the object is worth less than the story attached to it.
That is a meaningful difference from how the rest of the knife market works. Production knives depreciate the moment they are used, and most modern folders lose value as soon as a new version ships. A knife becomes unsellable when it stops being inventory and becomes a record of something. The collector who posts a scratched user with no resale value and says it will never leave is describing a completely different relationship to the object than the one the secondary market assumes.
There is a practical side to this as well. Knives that are never sold are also rarely maintained for resale, which means they get sharpened, carried, and modified. Several of the pieces in that thread were visibly worn, and the wear was the point rather than a flaw.
For anyone building a collection, the thread is a useful calibration. Buying for resale and buying for keeps are different hobbies that use the same objects. The knives that hold value are usually the ones with a known maker and a clean history; the knives that matter are usually the ones with neither.
Verdict: if you can name a knife you would refuse to sell, you already know what you are collecting. If every knife in the drawer has a price, that is a portfolio, not a collection.