If I'm dealing with someone I have never heard of before, I usually request them to complete their end of the transaction first. If it's someone I know or who I see frequents NS, I'm fine with paying them before they ship.
Although we have seen this doesn't always work
Dude, joke's totally on them, you should make a t-shirt that says 'all you fools suck' and on the back it can say 'I rule coz T-dawg said so' and yeah, you'll be rockin it shibby, new steeze brah, wikkid! ~PhattTim