I would think they want to double check that it will fit your car.

I'd agree - they want to check what the car is and that the liner will fit your car.

So they could construe that from the Numberplate - but you could just advise the make and model of the car instead if you wished.

Or you could just advise you've checked that yourself!

I should think it's because it's the only way the seller can confirm the buyer has purchased the correct item for their model of car - and will reduce the number of returns!