The word “knei” also means to acquire, to make a kinyan. When a man marries a woman he makes a kinyan, he is acquiring a wife. She is not being sold to him (though I bet a wife or two has felt that way in the past…). So one should regard a good friend as he does an important and crucial acquisition in life.
Actually, it is a ?????? in the ??????? (throughout ???????, also on :????? ?) how far the ???? goes, and is there a monetary ownership that a man has in his wife.
But you are right, that ??? does not mean necessarily to BUY, but means more accurately acquire.