I buy my knives for the pleasure in the hobby of collecting. I don't sell my knives, although I have grabbed specimens out of my collection to gift to others.
I did sell off a previous collection I had, which included knives, but it was more of a "get rid of" situation because I was getting divorced and mostly just temporarily lost my interest in the hobby.
As for making money off of them... I am certain it can be and is done, but there are a lot of variables.
Example one: You buy an older 50 to 100 year old knife. The brand/maker is known. Let's say it's one that is rare to find and it's condition is excellent to almost mint. Sure, assuming you paid a fairly low price for this knife, you should be able to sell this knife for a good profit. Only so many were made, few are found, and it's great condition is even scarcer.
Example two: You recently purchased a nice knife (new or used) that was made 20 years ago. It's in new to maybe great shape, but many were made. You paid the going rate for the knife. It's not rare, it wasn't cheaply purchased, and it's of fairly recent manufacture... So likely not a money maker there.
Too many variables to list, but I think that it simply depends on what you got, and how sought after it is.
Heck, it could have been free, you having found a Bowie type knife in the dry attic of your newly purchased house. The knife was kept in a box, and it seems to be in 95% condition. You take some pics of it, place them up on an online forum, (let's say Bladeforums), and many chime in to tell you that your Bowie was likely made during the years 1855 to 1865 in Sheffield England, and that it's a beautiful specimen likely worth a good amount of cash. You wind up getting more info on it, and wind up selling it for 10,000 U.S. dollars.
Almost any knife one has today, new or old, that is in nice shape and handed down, will someday be looked upon as a "back in the day" knife. So, while you may not make any money off of them, feel content that maybe some of your great great great grandchildren will have some of your knives, and that some will be worth a pretty penny in that future time... Of course these offspring may not know what a penny is/was, lol!
