Laura, it's a known scam, you can check Craigslist FAQs, but I've had this tried on me. The scam is basically in this part:
" i anticipate that a cashier check or certified check will be sent to you via express mail or ups as the mode of payment,"
They have no intention of buying or sending anyone to pick up your item. Instead they send you a cashier's check for more than your price, so if you're asking $100, they send you $3000 cashier's check. When contacted they'll tell you it was a mistake, but to go ahead and deposit the CC and then wire them the difference but keep an extra $100 for your effort. The idea is that you'll wire them $2800 directly from your account.
Due to how messed up our banking system is it can take banks 4-6 weeks to determine that the Cashier's Check is a fraud, at which time they'll reverse the $3000 deposit that was made to your account. Then you'll be out not only that $3k, but also the $2800 you sent by wire. The bank will do nothing to protect you.
I worked for First Interstate for two years as a Senior Teller way back in the early 90s. Back then Cashier's Checks were useful, now they are worth less than money orders or cash.
If you sell on Craigslist then you should only do so for cash, and you should state so in all of your ads in bold letters. I go so far as to have a blurb I put on all my ads, "Scammers beware, this sale is for cash only. No cashier's checks, money orders, personal checks, or credit cards will be accepted."
Then go by Office Depot or Office Max and pick up some currency pens for about $3ea, then kind you swipe across cash to determine if it's counterfeit or not. Counterfeit Detector Pens