One way to get even with them is to string them along, make them send you their counterfeit checks via FEDEX, and see if you can get their local cops to nail them. There was a thread on these guys awhile back in which there were links to websites that detailed how people did that.
Once you've got the check you can just tell them that you won't send money of the item until you see the money'd deposited in your account.
I think that if the deal's got this level of complexity, such as someone who wants to send you a lot more money than the item you're selling, & have you send them the remainder, should raise suspicion...
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse