You are absolutely right. I did do that back in the day. Looking back, I think it was a mistake, and I do NOT have such a list today, but that doesn't change the fact I did that, you are correct.
Today there might be people who don't get their parts until I am paid in full, but I can't think of a single person I would refuse to sell to.
Maybe in time you will feel that way also. We mellow as we get older.
But again, you are absolutely correct, that is a big part of why we work for ourselves, and you are well within your rights to run your business as you see fit..

On a side note, before I forget, congrats on the accomplishments this year! I have always loved your car!