I can almost guarantee that it's dodgy.
The guy's Asian and from West Yorkshire. I'm Asian myself and all those boys do is get a stolen car, switch around the plates, change the chassis number and flog it to a 'Southern nancy'. I really do despair for them. Young Pakistani lads from Leeds/Bradford seem to have a penchant for this kind of stuff.
Having said that, it might be genuine, but like everyone's already mentioned no one will really go through the trouble of converting a standard A3 to an S3. He also makes no mention of mileage or mechanical condition and there's no road tax or MOT.
Perhaps I'm making generalisations, but I'd be very, very cautious about buying a car like that.