Don't be too trusting of people you don't know
Between all the hacks, account compromises, phishing scams, and other despicable things going on, online trust is at an all-time low these days. Between scams like those, and the in-game scams that can happen in MMOs, you're right to be wary of strangers. Of course, in your real life, it's good to be wary of strangers as well. Keep your passwords and your personal info safe, and if someone offers you something that sounds too good to be true, remember that it probably is.