The certified check is a phony. Its good enough to get through the first round of checking, so your bank cashes the check in a few days to a week, your bank thinks its good, so you think its good. You give them the merchandise.
Meanwhile the check winds its way back to the issuing bank in a couple of weeks to a month, which detects the fraud, and the transaction rolls back at your bank, the check bounces, meanwhile you've given them the merchandise because you thought the check cleared already.
Randy
Were I inclined towards cynicism, I'd suspect that the bank knows it's no good, and takes it because they collect a fee when it bounces.
Just have to be headsup on these things and read the obvious sings of scammin.