I haven't seen a certified check since the 90s. I doubt many financial institutions are doing them anymore. For a check to be certified, a customer would take in one of his existing checks into the bank and the employee would stamp it certified and move the money into an internal account of the bank's.
Usually when someone says certified check these days they really mean a cashier's check. And the advantages are the same. The money for the check is set aside into an internal account at the bank. That means it can't bounce. Also, the person giving you the check can't change their mind about a transaction and put a stop payment on the check like can be done with a personal check.
The problem, as has been noted, is a counterfeit cashier's check is worthless. And just because your bank initially accepts your counterfeit deposit (likely without a check hold) doesn't mean it won't get returned days later.
When deciding how to accept funds from someone for a large transaction, I suggest asking yourself the following questions:
Do I know that this person is not trying to fraud me? If you don't know, you might want to ask for cash. Alternatively, do the transaction at the bank so you can see the teller preparing the check payable to you. Eminence suggests calling to verify. That can work but sometimes can be a pain in the ### to get through on the phone.
If you're confident the person isn't a criminal, next ask yourself if you know if the buyer is financially responsible. If you don't know, ask for a cashier's check or money order so it won't bounce. If you know he's fine, just save him the hassle and take a personal check.
Lastly I wanted to address this:
Be leery of credit unions and other smaller banks. Their intention is to deceive.
This is stupid. A real cashier's check from Chase or Bank of America is just as good as one from a smaller financial institution. Perhaps what he means is to use some common sense. For example, if you and the buyer are from Arizona, does it make sense that he'd get a cashier's check from some bank you've never heard of in Georgia? That's a red flag that maybe it's counterfeit. However, if it's from someplace small but local, that seems perfectly reasonable.