Selling My Xbox 360.................?

I was selling my Xbox 360 and some weird guy from Nigeria wanted to buy it, so i was only charging $275 and he said $375 and then He Said $500. and i was going to send it to him then outta nowhere he offered me $1000. So is this a scam or not???Please Answer. I need help.!. He sent me an Invoice for $1000.........should i ask for an advance or collateral.?.