When confronted with a scammer trying to pull the Cashier's Check (or any non-personal payment instrument) scam, tell them that your financial institution WILL NOT accept their paper for deposit and that it has to be sent in for collection.
True, a collection will cost you some $$, but, if the scammer's paper isn't paid, you're not out big bucks.
Alternately, you can ask your financial institution if the scammer's paper is valid. If it isn't, report him (or her) to the Postal Inspectors (mail fraud) and the FBI (financial crimes
Hopefully we can make at least a few scammers uncomfortable.