That's a typical Nigerian Scam.........
They try doing it to me at any time I've tried selling one of my guitars via on-line sales websites. For every properly interested buyer I get about 20+ e-mails from Nigerian scamsters.
The trick is that they pay you via an EXCEEDINGLY good forged cheque, and you then cash that cheque into your account. The cheque is a very good forgery so it'll clear into your account. They then say that they will handle the shippment (Why? FedEx, UPS, DHL etc etc are exceedingly good at delivering things worldwide) and the cheque they've sent you is always for more than you wanted for the item. This difference is to be paid via cheque to the person who comes to collect the item from you.
What then happens is that the person who's bank account has been debited with the forged cheque complains to their bank that they never signed the cheque. The bank then refunds their cash and goes looking for where the money got to, and therefore takes the money back from your account. You've actually signed the cheque you gave to the scam artists, and as this whole process with the bank reclaiming the money takes so long your cheque will have been cashed, and the money taken out of whatever account it was paid into.
You've then lost your guitar, your cheque for the item, and you have no record of where anything went because you didn't ship the item yourself to a recognised address. All you have is an e-mail address which will always be a webmail address, and will always die straight after they get your guitar.
They're scum and they've taken a lot of money from a lot of people, and I only avoided a similar scam on a racing bike I tried to sell via the sales part of the British Cycling Federations webpages because of one of my firends who works for the local plod. He tipped me off, and we tried to keep the scammer on the end of the hook by pretending to go along with him, only to find nothing to get them on in the end.
