1) There is a difference between a certified check and a bank check. A certified check is drawn on your account, and the bank had restricted those funds as to certify they are available. A bank check is issued by the bank. Both can be fraudulent.
2) While it might take a while for a check to clear through the system, (speed has greatly increased in the last 1 or 2 decades) you are still able to call the issuing bank and verify that in fact they did issue that check for that amount. Yes, big bank checks clear faster than small bank checks. These days banks present an image of the check for payment, the paper follows later.
3) Talk to your bank about security in place as to receiving a wire transfer from an unknown person within the US. I think you'll find that it is exceedingly difficult for someone to request a wire out of your account without being you. Banks are increasingly requiring verification that originates from within the bank for any wire over 4 digits, likely any wire at all. In other words, the bank must call you, the bank must originate the call, and the call must be made to a phone number previously established to be your phone number. You might be required to appear in person to originate the wire.
There's a ton of fraud out there, and new ways to defraud banks are happening every day. They spend large amounts of time on this.
A bank check that you call the issuing bank and personally confirm, and a wire transfer, are still both the most secure form of payment. Cash can be counterfeit or stolen, and carries risks as well. The secure way to handle cash is to take the bill of sale to your bank with the buyer, have the bank count and accept the cash, and then give the bill of sale and title to the buyer. The risk of the cash then resides with the buyer until it is accepted by your bank.