They are fishing for people who are willing to pay online for the vehicle without seeing it first (no, neither would I, but people do).
The story may be that they are a dealers and have taken the car in p/x and need to get shot of it quickly, they have lots of interest, in fact someone is on his way now with cash, so if you want it you have to pay a deposit (lets say £500), which is fully refundable if you are not happy after checking the vehicle over - payment has to be via bacs or similar, moneygram etc.
Its not really aimed at someone in the UK, more at people from, for example, Cyprus, who arent in a position to check its genuine.
It all hinges on the car being a bargain price, and there is an element of rush, often they ask for email contact only (on ebay look out for the ones that say email contact only, no contact using ebays mail system.)
Its all fraud. Even when you pay by bacs, the account you are paying into will be related to an identity theft.
The number on autotrader is a filter number (maybe even supplied by autotrader) which diverts to an unregistered pay as you go mobile.
Pete
Almost worth buying as a spares bin & only 90,000 miles 
