I've received offers for such advances multiple times via Craigslist. It's a well advertised form of scam and has been plaguing the site for perhaps a decade... Craigslist is primarily meant as a way to connect people for local transactions - hence the requirement to post advertisements within a region. Simply don't reply to any "buyer" who wants to do anything like this... It's almost always fake, and it's frequent as well! Wait for someone who wants to seal the deal in person, or at least talk to you on the phone and chat about the transaction.