If PayPal for some reason removed chargebacks tomorrow (wont happen, but entertain the thought), how many of these threads would still pop up? Next to zero.
How many would make excuses as to why they suddenly cannot pay even though they just 24 hours ago claimed to have thousands?
The ugly truth is: The vast majority.
Don't trust unknown people, ever. Only deal with foolproof methods. Trust large instances that have little incentive to steal your money & process millions per day, like MtGox. If you do trust an individual trader, make sure he has a reputation to lose, and even then preferably deal in small chunks to minimize risk.
Bottom line: Scammers aren't smart. Very few, if any people, are falling for these 'nice clueless guy new to bitcoin, want fast btc with pp' acts.