Personally I have posted about being careful to avoid these types of scammers, when someone is looking for a specific part to buy. This includes banning whatever name the poster is using to be on the forum. Unfortunately, they start another account and make the same offer: contact so-and-so at gmail or some other service, they have what you want.
Buy only from well-known members who have been members for years, and who have a history of successful sales. If someone says, "contact
[email protected], he has your part,"
don't!
I wish there was some way to prevent these criminals from joining and trying to scam people, but we do what we can when it's brought to our attention. That's banning the person. People who are more-into I.T. than me know how to access the internet addresses used, and how to compare them to known scammers, same goes for the email addresses.
Be careful in your transactions. We deal with scammers when we find them. If you are approached, report it and they will be banned.