They send this to the seller and buyer, and ask how much you paid for the car.
Then they match the two, and you had better make sure you both put down the same price. If not, the tax dept. starts investigating for tax fraud because you declared a lower price than you paid to save on taxes.
I have actually received a call from a seller once, saying "what $ amount do you want me to put down"? He just wanted to make sure that if I under-declared the selling price, he was putting down the same amount.
If you declared the right price when registering the vehicle, you're fine. If not, you might want to call the seller NOW and make sure your stories match!
By the way, you'll have to sign the form, right under where it says "by penalty of law.... false statements... sending you to pound me in the @$$ federal prison... penalty for perjury... etc".
So you will get nailed (pun intended) if those numbers don't match!