Escrow if you're feeling un-easy.
If not, Paypal. Get MSN/AIM address if possible to discuss etc. (Be sure to check background info on seller throughly.)
Its normal for you to pay, then domain to be pushed (Like a sales assistant and customer) but if you're not comfortable with that, ask for 1/2 first 1/2 after.
But beware, going for paypal; support is terrible!
Ta-ra.
Flubber