To state the obvious, this is a common scam.
Up here, it's an organized Nigerian group (no offence to any Nigerians on this site, but I'm just stating facts). They steal mail from boxes or mail trucks in industrial areas, and obtain company cheques (or checks in American spelling). They then try to either buy items, or just solicit people in chat rooms, in order to cash the cheques on their behalf.
They buy a $500 Corbin seat from you, take a stolen cheque, laser out the name of the payee, put your name in it's place, send you the cheque for whatever the amount, usually way more that the purchase price. Then they tell you, Oops, that's the wrong amount. Please cash the cheque at a Money Mart, and send me the difference back.
You'd be amazed how many people fall for this, and even fall for the other version-"I am the son of a Nigerian Prince who was ousted from power. I'm trying to get our money out of the country, so I'll send you a cheque for $10,000 in your name, then you send me $8,000 back, and keep $2,000 for your troubles". Yep, a sucker born every minute......