Tax period set as quarter for dealers, with cessation rules, turnover exclusions and documentary proof requirements. The rule sets the default tax period as a quarter. Cessation of a dealer's tax period occurs on the date specified by the Commissioner for cancelled registrations, on the date of death or winding up, or on date of cancellation otherwise. New registrants have a quarterly tax period from the date of liability until the end of the registration year. Turnover for tax-period purposes excludes sales of capital assets, winding-up sales, and sales constituting permanent diminution; adequate proof comprises annual audited or certified accounts and income tax returns for the three preceding years.