mailing, check, and/or shipping
It's as simple as that. Craigslist is ONLY for face-to-face, cash-on-the-barrelhead transactions. As soon as "mail", "check" or "ship" comes up, block the "buyer". It is a scam.
Of course, your guy dinged all 3 bells.
If someone wants to do that type of transaction, and you believe they are real, you should force them onto a platform with proper anti-scam and seller protections, such as eBay/PayPal. And you need to play savvy and by the rules there, too. The methods used by Craigslist scammers do not work on any eBay/PayPal who is savvy and following the rules.
Of course the same is true for Craigslist, the rules plainly say "Local/face to face sales only".
How it works.
The scammers are on Craigslist because that works, because 1% of people will be fooled and agree to absurd transactions.
The "check scam" is that the check is forged in a way that will take a very long time to bounce. However the bank only places a hold on the money for X days (that's the time most checks take to bounce). Consumers think that the hold release means the check cleared. It does not. Meanwhile the scammer is telling you "Look, the check cleared, send me back some of the money!" And gives you an irreversible method like Western Union. Then the check bounces, and you are out the money you sent onward.
There's some excuse for overpaying, and then the scammer says "Pay my shipping company when the item is picked up". That gets revised to "Oh, my shipping company needs it in advance, can you wire it to Western Union 123-456-7890111".
And then the shipper never shows up. The payment you wired was the endgame of the scam.