Is A Cashiers Check Safe? How Long To Clear Bank?

They are usually treated as cash since most banks clear them instantly.
However, banks are permitted to take back money from a "cleared" check
one or two weeks later if subsequent processing finds it to be
fraudulent. Because customers believe the checks have been found valid
and have been converted to cash in hand, customers are readily
defrauded by schemes which ask them to part with goods or a portion of
the money if it is cleared in a timely manner.I would not take it...

Money Order is safer than a cashier's check
 

Require that they purchase a postal money order instead, and do not give or ship them the item until you have cashed it.