I had the same thing happen to me. We were selling an older pop-up style camper, for around $1000. They sent us a check for $5000 dollars and said all we had to do was send them a check back to them for the difference. We took the check to our banker and he let us in on what the scam was going to be.
If it sounds to good to be true it probably is.
There is alot of these types of scams out there so beware.