People get scammed because they're lazy or desperate.
Lazy: To lazy to actually do the leg work to check if someone is legit. They come in here and without checking refs or even doing a simple search of the sellers name they send them money. That's how the same scammers keep scamming, over and over.
Desperate: If they actually DO the leg work and find out the seller doesn't have any refs yet, but refuses to ship first, they won't walk away, they'll give in and just pay for it. If I determine a seller isn't trustworthy enough (judged by a combination of join date, post count, refs, & feedback threads) then I will ask them to ship first. I generally consider myself a trustworthy member, (been here a long time, lots of positive transactions) so they should have no reason to refuse...if they do, then I walk away, no questions, no negotiations.