Tell them you will accept only an USPS money order.
Then, cash the MO when you are ready to mail the item ... if the MO is fake, don't ship!
Even better, follow the Craigslist advisory:
Avoiding Scams
Deal locally, face-to-face —follow this one rule and avoid 99% of scam attempts.
Do not extend payment to anyone you have not met in person.Beware offers involving shipping - deal with locals you can meet in person.Never wire funds (e.g. Western Union) - anyone who asks you to is a scammer.Don't accept cashier/certified checks or money orders - banks cash fakes, then hold you responsible.Transactions are between users only, no third party provides a "guarantee".Never give out financial info (bank account, social security, paypal account, etc).Do not rent or purchase sight-unseen—that amazing "deal" may not exist.Refuse background/credit checks until you have met landlord/employer in person.
https://www.craigslist.org/about/scams
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