if you get scammed its your fault for being stupid enough to get scammed, trade with people you trust.
This is the stupidest thing i have ever heard in my life This is one of the smartest things. Just because you get scammed it does not mean you are stupid it just means you trust the other person. I got scammed by someone i have known for 2 years in game for 3k so your point is invalid.
You cannot, just say don't deal with anyone but your "friend?" to make friends and to trust them you need to speak to them, and to gain trust buy stuff from them.. and both gain trust in each other, However not everyone is as loyal as you think just naming them a "friend" doesn't mean there trust worthy, they may be there for you sometimes but they will most likely dog you at some stage, when you have the money and they have a chance to take it, if it's a big amount of money.