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I just don't see a real solution to the underlying problem with newbies wanting to "invest" into something they don't yet understand.
If in doubt, trust a Donator!
Seriously though, you're absolutely right, donating money to the forum is one thing but it doesn't really say a lot about the trustworthiness of said person.
At least, it makes scamming expensive. If you paid 10 BTC for Donator status, you're probably not going to throw away your reputation for anything less than a few hundred coins. Also, if you already have that kind of money, you're just less likely to scam in order to make some more.
The best advice for them is not to make deals with anyone as long as they're not able to figure it out for themselves.
But will rubbing their nose into such a "prime directive" really make a difference?
I've just seen so many few-day-old accounts throwing their first coins at whoever offers running a cash-printing-machine for them (i.e. ASIC group buy) that I just gave up on them.
Probably the best thing would be a change of the Trust system where everyone's trust rating is reduced to -100 for any new user before they hit something like senior status. That way, they'd at least see all of us as "Warning! Trade with extreme caution".
I really think that could work, ridiculous as it may seem. It'd certainly ring some alarm bells.
OTOH, if all you see are crooks, the effect would probably wear off just as easily as you get used to clicking away those nagging warning popups on Windows computers
You may factor in whatever you like, there is just no way to determine if a specific user is going to con others in the future. A long post history, great trust ratings, Donator tag, whatever, it just doesn't mean anything. IMHO, there is no solution to that problem.