Its a well-known scam and its described all over this site.
The way it works, if you agree to sell it to him he won't be able to pick it up. His payment will include extra money that you are to wire to a shipping firm he has chosen.
If his payment is supposed to be by Paypal he'll send you and authentic-looking notice that claims to be from Paypal saying that his payment is in your account without actually putting money in your account. Or he might pay by a fake check. The check is a well-made fake and can take days or weeks to bounce.
Your wired payment to the shipping company clears, the buyers payment bounces. Your bank makes you pay back the entire amount of the check plus a fee for depositing a bad check. The buyer and his friend, the fake shipping company, share the shipping fee.