This scam relies on a misunderstanding of how banks process checks. Under federal law (in the US), banks must make deposited funds available within days, long before the check actually clears the issuing bank.
How it works
You sell an item online. The buyer sends a check for more than the asking price and asks you to wire the excess to their "shipper." You deposit the check, see the money in your account, and wire the difference.
Weeks later, the check bounces. The bank removes the full check amount from your account, and the money you wired is gone.