The way the scam works, the buyer offers to pay your full price for the item without ever looking at it. He wants the item shipped to him. The buyer's check to you includes extra money that you are to wire to a shipping firm he selected.
You deposit the check in your bank account, then wire the shipping fee to the shipper. Your wired money clears immediately. The check is a well-made forgery. It bounces, but that may take several weeks. You owe your bank the entire amount of the original check, including the money you sent to the shipper, plus a fee for depositing a bad check. The shipper and the buyer vanish. They keep the shipping fee you wired them.
The only safe way to sell on Craigs list is to insist on cash only, to be delivered in person when the item is picked up. Even then you'd be wise to use one of those markers they use to check if money is counterfit.