How do I ensure that this Craigslist buyer doesn't screw me over?

All good scam artists "seem legit from the e-mail correspondence." I would not have accepted a check in these cirstances. I'd say "no money, no mattress." Send the movers away until the check CLEARS. He says he mailed the check, but didn't actually mail it until he was sure that it would not arrive in time to clear the bank by the time he gets the mattress. He says "oops, I sent too much. Just send me the extra when the check arrives." So not only does he get a free mattress, he get you to pay the movers for coming to get it. Don't send the extra until the check CLEARS. A check is just paper, and I think you've already been screwed.