D.C. Code § 47-2203

Every vendor engaging in business in the District and making sales at retail shall, for the privilege of making such sales, pay to the Collector the tax imposed by this chapter. At the time of making such sales the vendor shall collect the tax from the purchaser and give to the purchaser a receipt therefor in such form as prescribed by the Mayor. For the purpose of uniformity of tax collection by the vendor engaging in business in the District and for other purposes the provisions of §§ 47-2003, 47-2004, 47-2009, and 47-2010 are hereby incorporated in and made applicable to this chapter.