I just had the same problem 15 mins ago with the Rondells I am selling. This toolmuncher takes the Buy It Now option and then emails saying he is sending a cheque for $5000 and I *MUST* (LOL) despatch the wheels and his change within 24 hours or he will leave bad feedback!!!
Report the scammers to eBay ASAP and you get your fees refunded if their contact details don't check out.