Scams like this are especially prevalent on Craigslist. A few months ago, my dad put up a listing for an electric guitar he was selling. Someone replied within a day, claiming they lived in Nigeria or South Africa or some other country and had no way to pick it up right then. He asked that my father hold "the item" (big red flag there, when they refer to it as "the item") until his "assistant" was able to pick it up for him. In exchange for holding it, he offered to pay more than the asking price for the guitar.

Nobody that can afford their own personal assistant buys used stuff off Craigslist. This is a very popular method for scammers. They even send checks which appear real but aren't, so bounced when cashed - meanwhile, the scammer has made off with whatever you were selling. This is precisely the reason Craigslist posts warnings everywhere about scams and suspicious emails. Scary stuff.