Thank goodness it is a hobby for me (albeit an expensive hobby!).  I try to never look at it as an investment.  From that perspective, I tend to smile if I decide to sell a knife in which I've lost interest and receive more than I paid for it.  On the other hand, as a hobby, it doesn't bother me at all if I sell a knife for less (even much less) than I originally paid.  My father-in-law lives for collecting coins.  Neither his son nor daughter (heck, nor his grandkids) have an interest in coins.  He is always telling me how much this coin or that is worth.  I always respond with something like ... why don't you sell it (or them).  His response is always the same ... they are an investment.  Oh ... he is 90, by the way!  I guess he plans to live forever.  "Things" are only an investment if you have a plan for selling them.  He doesn't quite agree with that statement!  His coins are his and he has no plan to sell them, period .... however, he will never be caught calling this a hobby.  It's an investment, of course!  

You learn the large amount of time required to sell knives to upgrade a collection or just get the cash. Its best to buy right but that takes experience. The appreciation of knife values to me does not rise as fast as an investment in a S&P 500 indexed fund. Later in life there are few purchases that give me the satisfaction of a knife for the collection. Some collectibles have gone way down in value -for instance Beanie Babies---Knives in some way will keep value as they are beautiful or exhibit something intellectually obvious without your knowing their historical background. Knives have different uses over time even in life saving and taking experiences.

Paul, your right in that if you buy a knife right it will never lose value.  Buying it right takes knowledge or at least it did for me.  Being a history buff finally came in handy LOL