The Collaborative International Dictionary of English v.0.48:

mail-order buying \mail-order buying\, mail-order \mail-order\n.
   The buying and selling of goods to be shipped from the vendor
   through the mail to the purchaser. Information about to be
   purchased may be found in catalogs, advertisements, on the
   web, etc., and purchase orders transmitted to the vendor by
   mail, telephone, or internet connection.
   [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

mail-order buying
    n 1: buying goods to be shipped through the mail [syn: mail-
         order buying, catalog buying]