Its the Overpayment Scam.  The buyer will offer to pay your full asking price for whatever you are selling.  He wants to send you a cashier's check for the amount plus extra money for you to pay the shipping company selected by the buyer.  You cash the check then wire money to the shipping firm.  Your wired money clears instantly.  The buyer's cashier's check bounces, but it takes a week or more.  Your bank makes you pay back the entire amount of the check plus an extra fee for depositing a bad check.  The buyer is also the shipping firm.  He keeps the shipping fee.

Another variation has the buyer's money come to your Paypal account.  You get an email from Paypal the the money is in your account, but its fake.  You pay the shipping firm, then discover that the buyer's money is not in your paypal account,

Congratulations on not falling for it.