In the circle of sharpening knives, they are called foreign garbage. Most of these knives were eliminated by Japanese chefs. They buy the whole package and then grind it.
So if you want to buy sharpening technology, you can buy a few to play with, and then try to sell them after grinding. If it is for your own use, I suggest you buy a new knife, especially Sande Niu Dao, which is used very frequently. It is really not recommended to buy second-hand knives.