Vingry: I see what you mean. Although I have to argue that there really aren't many tips and tricks when it comes to protecting oneself from scammers apart from searching for the names of the users in the forum, looking at their rep (ineffective), and questioning the screenshots that those users send to you. I'm referring to those screenshots which can be proven to be a product of some edits from a picture editing software with further inspections. Other than these three, I don't think I can come up with anymore tips and tactics. Any tips to look and study in to the patterns of how scammers conduct their scams would only lead to mistakes and like what you said, shaming users who might actually be innocent.
If it quacks like a duck, it's a duck...
In other words, if the offer is too good to be true, it's a scam. But too many people seem to fall for it anyway "because it was too good a deal".
The real questions to ask, "Why does this person just happen to have an extra code lying around of the exact game you want even though it has never been highly discounted?" - They will try to convince you it's some special promo code, don't believe them.
And "Why are they trading away a game worth far more for bundle leftovers?" - They will try to tell you they "missed the bundle" but codes can be had elsewhere for cheap. Again, don't believe them.