I would imagine that this scam is a way for them to launder stolen money by tricking a (stupid) 3rd person into doing the dirty work of actually handling the most risky part of the transaction. That seems particuarly likely in Chony's scenerio where they did actually send a cashier's check as was discussed. The fact the police showed up 10 months later at the marks door almost proves the effectiveness of the scam in putting a dupe in between the criminal and the original victim of the theft.

The other possible scam is that the offer to send the check with the excess money is simply a confidence builder and the person accepting would soon find another 'twist' that would require some sort of good faith money.

Regardless this is definitely a scam and probably should be reported to Ebay.