You've got your model cockeyed. TJMaxx and Macy's are retail outlets similar in the publishing industry to Borders and Barnes & Noble. And yes, Borders and Barnes & Nobles have buyers, who are approached by the marketing department of the publishers such as Penguin or Harpers.
Comparing publishing to clothing is somewhat difficult. But an editor would be roughly the same as a designer for Wrangler, Hanes, or Levi-Strauss (hey, I'm a guy, you're lucky I can come up this many names). Agents are a screening device because of the sheer number of proposals hitting publishers. And agents contact editors.
Best of luck,
Jim Clark-Dawe