• Get everything in writing. Even if the person you're doing business with is a friend: with everything in writing, should something go wrong you know how to fix it. Don't ever go into a deal with the mindset of "We'll cross that bridge when we come to it." Trust me: when to get to that bridge, it's likely that it will be on fire.
• Sell what you trust. No matter what the product, if you sell it you're responsible for it so if it's crap, the customer is going to think bad thoughts about you, not your supplier.
• Sell what you know. Example: If you don't know the difference between a caplock and a matchlock, don't sell reproduction firearms, for a similar reason to the previous: the customer is going to come to you for information and advice, and they expect you to have it now.

So, then the money starts rolling in...