I was approached with this deal about a year ago. Although I knew it was a scam, I couldn't see the catch since the guy said he'd send me a cashier's check from a US Bank. Turns out you can stop payment on a cashier's check, which is what they do. I always thought they were good as gold.
The ex-husband of one of my employees actually fell for the deal. Cashed his cashier's check, mailed off the difference between his parts, which were never picked up, and the total. Then the cashier's check bounced cause it was a stop payment and the bank sued this guy for the balanace. I would suggest running as fast as possible.