Maryland Tax - General Section 11-504
§ 11-504.
  (a)   A vendor shall keep:
    (1)   complete and accurate records of:
      (i)   all retail sales and sales for use; and
      (ii)   the sales and use tax collected; and
    (2)   other records in the form that the Comptroller requires by regulation, including bills of lading and invoices.
  (b)   A vendor shall make the records under subsection (a) of this section available for inspection and examination by the Comptroller at any time during business hours.
  (c)   A vendor shall keep the records required under subsection (a) of this section for 4 years, unless the Comptroller:
    (1)   consents in writing to an earlier destruction of the records; or
    (2)   requires in writing that the records be kept longer.