It doesn't really make a difference. When you're paying for an item from the classifieds with paypal, you can either
1) Send a payment for goods/services to his paypal email account.
or
2) He can send a paypal invoice to your email address, to which you'll respond the same way in option 1.
As far as I can tell, there's no difference between the two methods. The seller will pay the transaction fee and you'll attach the shipping address.
Whether you pay him without him sending you an invoice first or not, the transaction will be on record the same way.