I think it’s unlikely that the seller didn’t know but then again maybe they missed it as you did. However, while it’s not ideal, you may not have lost a great deal. It doesn’t take much for an insurer to write off a car, especially if it’s a few years old and, as long as it’s been properly repaired, there may not be much wrong with it.

You might want to put it down to experience and not dig further. However if you do want a view as to any loss you might have suffered then if you give more details (category of insurance write-off, mileage and price paid) there might be people on here who will give an opinion as to how good or bad a buy it was.