Anyone can list whatever price they want on a knife, or any other item.
If the market for that knife is high, the seller of course is going to list it as high as possible, and possibly drop the price if no one has shown an interest.
There are no rules here at BFC stating that you must list a knife at a certain price. It is the individuals choice, and if any of us disagree with the individuals price, we can either make an offer, or go do business with someone who lists a lower price.
Remember, we live in a capitalist society. We sell goods based on what the market will bear, and we buy based on price, value, need, want, or desire.
I hope this is the answer you are looking for.
Feel free to e mail me if you need more details, or clarification.