Dude, you lost out on a knife that you wanted... obviously at impulse purchase worthy price. Unless its super rare, if you needed the knife you would own one all ready. Dont waste your time and energy following this guy on other platforms, documenting his activities etc. All that is doing is building up stress in your life. Chalk it up to people are shady, especially when their only interests in the hobby are profit based.
The knife will come back around again, or odds are, soon you will move on to something else.
Best of luck.