Craig's list sale

I listed 2 cash registers on Craig's list for sale at $900 each. I received an e-mail saying that he would like to buy one & would send me a bank check. He was located in Fl. & said that after the check was cleared he would make arrangements to have it picked up. He also ask me to remove the listing from Craig's list. I said OK but which register did he want? He came back that he would take both. I was thinking this was a scam? I did not remove the listing but received a check for $3600 by express mail. Bells went off because I had heard of this where they send you too much money & then act like it was an honest mistake & because I had already deposited the check could I send back the difference. I was trying to figure a way to reverse it back on him but consulted with my bank & it's a lost cause. He was from Florida but the check was from Texas. We didn't deposit it & never heard from him.