I have a wuestion about getting scamed on paypal. how can you see a scam coming?

What my problem is, is I have a guy trying to buy my dirt bike off craigslist and is says he's an oceanographer. He said he would pay $3300 for my bike and take care of the shipping. The shipping guy is located in california and his payment of method was western union another wireing company like paypal. These sites are both secure but the shipping was $900 that I payed and the guy send it to my paypal account along with the $3300. Paypal sent me an email confirming this but I needed to send a scan receipt of my purchase for shipping to paypal. I did this and then they said they would credit my account with in 6 hours I get another email 6 hours later saying I have to pay $1300 for an insurance fee for the shipper. I have to pay shipping fees because of his job and he can't deal with it. Then paypal sent me another email saying the total amount of $5600 will be credited to my account once I pay that other $1300 and send thbm another scan receipt. Then I emailed paypal like 5 times bitching at them and they finally said they were going to monitor me and this guys transactions to make sure its legit. Someone help me please with advice or if you know its a scam or if I should continue and then sue paypal and this guy if it is a scam. Email me please at lreitzner@hotmail.com thank you