Mr. Barnett owns the Barnett Mercantile, the shop in Strawberry that the Logans visit and that T.J. and the Simms brothers rob. Mr. Barnett dies in the end from getting hit on the head by a Simms brother.
Like many of the white characters in the book, Mr. Barnett is racist. Openly, gleefully racist. He interrupts his transaction with T.J. several times to help out white people, and, when Cassie calls him out on it, he calls her a "nigger" (5.62) and demands that she leave his shop.
As if that weren't enough, he's also dishonest. T.J. tells the story of how Mr. Tatum previously had a dispute with Mr. Barnett about an order to which he (Mr. Barnett) creatively added items. Did we mention he wanted Mr. Tatum to pay for those added items that Mr. Tatum knew nothing about? And that Mr. Tatum was tarred and feathered by an angry crowd when he objected?
Thanks to Mr. Barnett, we can see that racism affects every mundane aspect of everyday life—even grocery shopping.