I had the same skam when I tried to sell my bike on ebay, the guy sent me a cheque, I was not fooled at all and wound the guy up for days, it must have cost him some money on the phone and I kept the cheque, one less to get someone else with,
if you get this scam tell them your pc is on the blink and to phone you, and keep them on the phone as long as poss, call it cheap justice
