Benjamin Clymer says:
Rule #5. Don’t be a fool. My final piece of advice to a beginner vintage watch buyer is always to assume the very worst of people....

I like people. I don't believe that most people have evil intent. When you assume the worst of people you will never have any friends.

My rule #5 would be: Don't be a fool. A collector's passion has no rational basis. Be cautious while you are being bold. Treat people with respect until they prove they don't deserve it. If a deal starts to smell bad quit before it ends up stinking. When something seems too good to be true use your head and not your heart. Almost all mistakes are your own fault.