Any retailer required to collect the tax imposed by this Act shall be liable to the Department for such tax, whether or not the tax has been collected by the retailer, except when the retailer is relieved of the duty of remitting the tax to the Department by virtue of having paid a tax imposed by the Retailers’ Occupation Tax Act upon his or her gross receipts from the same transactions. To the extent that a retailer required to collect the tax imposed by this Act has actually collected that tax, such tax is held in trust for the benefit of the Department.