When I bought my first few knives years ago, I had no idea I would end up with several hundred in my collection, so I didn't really stop to consider what and why I was buying.. Some of them were cheap, some caught my eye initially for their visual appeal, and some just looked tough and mean. In other words I had no real idea what my actual tastes were, or what I would do with some of these blades after the first thrill of ownership wore off.

Now that I look back I realize I had no business buying blacked out, fixed blade combat knives or some of the bone handled hunting knives I acquired. I am neither a hunter nor a military man, and they weren't really of use for what I eventually decided was my passion in knives, which you will learn below. As a result I own more than a few knives which I should trade or sell outright. Many of these knives were quality pieces, with nice sheaths, but for me personally, that doesn't do it.
I understand know that I should have been smart enough to take a little time after the first few purchases and carefully consider where I was going with my mounting collection. What I would like to present below are a few thoughts you may wish to consider before buying your next knife.
First of all, no matter what type of hardware you choose to collect, don't buy because of cheap prices. I found this out very early. The knives looked good in the pictures, but when I opened the box I usually got a crudely made knife with ill fitting scales, rough action, and cheap steel that wouldn't take a good edge, let along hold it. In reality I had just purchased a hugely expensive knife because it was useless for cutting or display purposes or anything else.
Second, sit down and consider what you really like in knives. Is it the visual appeal, the mechanism, the blade and handle material, the style, the size, the possibility for customizing, the intended usage, the collector value, the brand name or maker, the historical value, or a combination of several things.  Sometimes you have to buy a knife for a given reason and then carry it or display it for awhile to see if it stands the old test of time. In other words after a few weeks of ownership does it still appeal to you and would you still buy it if you didn't already have it. This can be a little costly, but you should learn quickly what you do and don't like.
 I have read the average purchased knife only lasts two (2) years. That is amazing, but it it's true, they certainly haven't talked to me or seen my collection. True, I have lost a few folders many years ago to various seats and sofas somewhere, but the vast majority of my knives are still around and in good condition. Some I just display, some I use, and some I do both. A fellow collector once told me he pulls one knife per week from his collection to carry, and then goes back and gets another. (I liked that concept when I first heard it, and have roughly adopted the procedure as my own since.)  In other words you should expect to have your knives for a long time, so you should buy something that will please you to carry over and over.
Thirdly, this brings up the question of buying expensive knives. Obviously when something is expensive for a collector, he considers it a little longer than he normally would before buying, but maybe for the wrong reasons. I have a David Yellowhorse 3 blade folder with all kinds of etchings, an inlay, and a dynamite wood and glass display case that came with it. For me it was the most expensive knife I had ever purchased at the time. I used to get it out of the case and actually carry it, but then several years later when I saw what it was now selling for, I quietly cleaned it and put it back in the case. where it has remained. I can no longer bring myself to use it because of its worth. I mention this because I think it should be one of the big considerations on any expensive knife purchase.  Consider carefully what you intend to do with this knife. If it's for appreciation and display, fine, but if you had thoughts of actually using it, you better make sure you can justify it to yourself after the box is opened. Again, know what you are buying a knife for.
Fourth, read a little about the knife you have your eye on before you buy. Are there any reviews? What do different purveyors have to say, and what are their prices and service policies for customers. What is the reputation of the company or maker?. What about the blade steel?
Fifth(and last), if you're like me, you have multiple reasons for buying a knife. If so, try to maximize the number of reasons in a single knife purchase. In my case, I love it when I find a knife that has great blade steel, possibility of customizing( I make custom handles as a hobby), a strange and exotic mechanism, and uncommon handle and liner material! .It doesn't get any better than that for me.
I hope these thoughts at least cause you, the buyer, to pause and take a second breath before taking the plunge on a new piece of steel.

Dr. K