How could this situation be a Craigslist scam? [Details Inside]?

So a guy on Craigslist e-mailed me back about an item I’m selling and would like for me to send him an invoice through PayPal. He provided me with his e-mail. My buddy tells me through PayPal it’s the safest thing in the world. My buddy also tells me if it turns out that guy is fraudulent that I would then be abel to still keep the money and they would go after him.

Any pros and cons here? Obviously I wouldn’t ship the product out until his payment has cleared. He claims he is in the army and wants to get the product for his fiancee. What are your thoughts?