lol this is the nigerian scam. I'm surprised you didn't know about this. It's been going on for a very long time.
Their are some variations, but ALWAYS, ALWAYS, will he say he will send immediate payment and after that you must immediately ship the item to his location using the shipping company he (sometimes) has vouchers for.
The very smart scammers will offer you a higher price, then tell paypal you scammed them and they gave you too much money on accident and wont get it back. You'll get no money, no product, and you'll be screwed.
Somebody tried to do this to me on craigslist. I went along with it and asked for their shipping address, and then gave it to the cops.