Perhaps he's not as dull as you think then? It takes 2 seconds to look on the internet to see the value of your car so thinking your getting a bargain by duping the seller in this day and age is unlikely, but not impossible.
He maybe, as said above, just rectifying an issue which is a good thing really or perhaps he's the type to remove good bits off the car just before you come to pick it up like a cat etc.