im guessing that he will "surprise" you when he has bought it and put it on the drive (then it will be too much hassle to refund and he will say he did it for you)
Yep definitely what cazzzz said, it'll be 'Look Darling, I've got a lovely surprise for you!' and then when you suggest that you might have liked to know before hand he'll sulk all day because you're ungrateful. (even though really he'll know he should have told you anyway, the ungrateful line will just be the getting away with it plan!)
Maybe it was only a casual enquiry on Mr WP's behalf, and the sales man decided to follow it up - I bet he comes home and says I saw a car at the weekend - do you fancy a look at it - on the other hand maybe he is thinking of financing it in your name, forging your signature and running away with the milkman - was it a camper van?
Book two weeks in Marbella and pay for it before he can buy the car
Open up bank account in your name and hive a decent amount off into it for hairdos, pampering and the like and tell him that identity theives have taken it
Do a Pauline Fowler and clonk him with the frying pan.