There are basically two major reasons for purchasing a knife. One is for your personal use and the other is for a collection. Of course there are other reasons such as a gift.

When purchasing a knife to use, there are some basic guidelines to follow. Be sure that you buy the right tool (knife) for the job. Don't buy a dagger to clear brush. A kitchen knife is not the best boot knife and a Kukri will not double as a utility hunter.

There are knives designed specifically for most tasks. Some knives are good multiuse tools that can be used for a variety of jobs. Camp knives are a very useful tool in the field. One can construct shelter, chop kindling, prepare game and perform many other outdoor duties with just one knife.

A medium sized Hunter or Sportsman can be used for basic cutting, preparing game, light chopping, self defense, etc..
Combat type knives are usually designed weapons of offense and defense. Typical uses are slashing and or stabbing. In a pinch a combat knife can be used for other purposes but in general a good "weapon" knife has a solo purpose.

There are utility and other knives that can be used in a combat situation. These are more of a "last resort" type of weapon that will suffice in a just use situation.

Long knives and swords can be both weapons and utilitarian tools. Things such as heat-treat, blade and handle length and shape will dictate the tools optimal use.

As for buying a knife for collectible purposes, you should buy what you like first and worry about realizing a profit second.