Auction sheet is basically the snapshot of car's condition when it was auctioned in japan. It shows the car exact physical condition, mileage etc on the auction date. Remember not all Japanese imported cars will have auction sheets since the cars which are imported privately don't go into auction and have no auction sheet. For those cars ask if you can see their pictures when the car was in japan b4 inport.
Export certificate on the other hand is the certificate which shows the dates, mileage and importer details at the time of import. It is the most important document required for car registration and u will need to submit it to excise for registration. So if you are buying an unregistered car Always get export certificate.
You dont need auction sheet for registration.