Do not give them any of your details, someone tried to pull this scam on me too. It sounded fishy from the start, if things sound too good to be true...they usually are!
There are different variations but they are all something along the lines of below (which I copied from a car selling website)
Quote:
Car Buying Scams
A number of users have contacted Autoweb regarding email scams they have received while using our website. Whilst these scams have only affected a small minority of our users it is a scam that has the potential to leave you thousands of pounds out of pocket.
The supposed buyer will offer to send you a cheque for an amount greater than the actual asking price of the vehicle. They will then claim that this is to cover the shipping costs and will later ask you to send a cheque back for the difference. The initial cheque they send will appear to clear - only later to find that it was a counterfeit and will be withdrawn by your bank. By this time it is too late and they have cashed your genuine cheque for the supposed over payment.