If you don't think that you bought their knife, you should ask for proof and a link to the page that sells the knife. Also, since you made your knife from scratch, it is yours to do with as you wish. Similar items abound in free stuff and for sale, and it is quite presumtuous of Mathias and Daniel Kruse to ask you to remove your free item that "competes" with their for sale item. If they claim that you copied their product, then a third party should examine the models and make that determination. You have made a lot of great free items, and you are always careful and respectful of the copyrights of others.