You absolutely can be, especially if you knew, or should have known, that the knife was likely to be used illegally. While a bit more extreme than your example

You absolutely can be, especially if you knew, or should have known, that the knife was likely to be used illegally. While a bit more extreme than your example, there have been multiple cases where the parents of a school shooter have been held responsible because they provided access to a weapon when there were warning signs. On the less extreme end of the spectrum, this is the same reason why you have to pretend that you are buying a "water pipe for tobacco" and not a bong if you don't want to get kicked out of the headshop (in places where that is still illegal).