The issuing bank can verify immediately, the receiving bank must wait till it clears the issuer. Bank issued MOs are more quick to verify than Western Union money orders which are the ones most often faked.
First of all it's not a Money Order, it's a Cashiers Check, huge difference. If the issuing bank verifies it, there is no reason for the seller to wait for it to clear, because it WILL, and that's guaranteed. A Cashiers Check is paid for buy the buyer, and is signed by the issuing bank official. A stop payment can not be put on a Cashiers Check.