So much wrong information in this thread. Seriously, do some of you people just make stuff up?
A Cashier's Check goes through the same banking system process that a personal check does. That means that it could take 3-5 days or more to finally clear, depending on where the Federal Reserve Banks the depositing bank and the drawing bank use. That assumes that both banks are in the U.S.
After you deposit a check, even if the funds are available, that does not mean that the check has cleared.
A Cashier's Check is generally not subject to a "Stop Payment." That doesn't mean that the check couldn't be stolen or forged. Keep in mind that "Stop Payment" is a special term - there are other ways to attempt to prevent a legitimate Cashier's Check from being paid, but the issuing bank could be taking a big risk by doing so.
Banks can't put Stop Payments on Cashier's Checks
Lost, Stolen, or Destroyed Cashier Checks
A good way to check things out is to directly call the bank that the Cashier's Check is drawn on. Ask them to verify the Check number, Account number, Remitter, Payee (your name), and amount. I fielded many calls like this when I worked at a bank. You can also ask your bank to do this for you.
Remember to look up the bank's telephone number. Do NOT call the number on the check. One of our tellers made this mistake. $$ walked away.
RossMan's right - wire transfer is the safest way to go. Buyer wires money (use Wire Transfer, NOT ACH) directly to your account. I think Wire transfer fees are around $25 at most banks.