I'm not good at teaching stuff but "By" is more for linking a subject. For example: "I came by car." or "He won the race by 3 seconds." "Bye" is what you say when you bid farewell for the day or the moment. hope it helps

I so not spell well. As a bad speller who also knows the meaning of words, I would say part of my spelling issue is laziness. In this case, I think I know that buy is to purchase while by is location. Good buy is a good deal and good by is farewell. Why would a bad speller like me know? I looked it up to be sure? Ill never tell. 88