As I understand it, this scam (and others like it), start off in this manner, they offer to buy the car at asking price or slightly higher. They state they can't come to personally view the car as they are (insert excuse here - oil rig worker, on the moon, in hospital etc etc) but they wish to purchase your car and they need to finalise the deal quickly.
If at this point you are foolish enough to give them your paypal details they then send you a fake paypal notification stating that the money has been transferred and it is in paypal's account ready for handing over. However they will also tell you that it cannot be released (and the excuses here can be anything i.e until freight, agents fees etc has been paid or cleared or arranged or converted - yet another excuse). At this point the scammer then ups the ante on the urgency of the transfer and states that if you (the seller) pays the freight, agents fees etc, they will refund you what you have paid (usually a couple of thousand dollars). They immediately send you another fake paypal notice informing you that you have another payment waiting for final transfer.
If at this point you are stupid enough to pay the freight, agents fees (insert excuse here for additional fee), they demand that it be paid by Western Union or some other non-refundable means to their account. Once they get the money, you then will fall into one of two categories, Number 1 is, you never hear from them again and you never get the money from paypal (because it doesn't exist) and if you contact paypal they will tell you both notifications you received are fake or 2, they say something has happened and that you have to pay more money to get the original funds cleared, but they will reimburse you and send you another fake paypal notification that the money is there - and at this point they try and scam even more money off you.
There are many variations to this scam, but it usually follows this path. Don't toy with them as it helps them to improve their scams by working out what tipped people off in the first place. The best way to deal with scammers is to not reply to them at all. Hope this helps.