Barney twice pulls the squad car to the side of the road to confront the farmers selling produce. When the car stops, it sits at an angle to the road. When Barney exits the car, the car sits parallel to the road.
The outside background, as seen through the door of the office/jail set, generally shows a column matching the columns of the exterior shots, but in a few episodes of Season 3, the background features a painting of the patrol car parked right outside the door and no column.
Street vending was allowed in the previous season, when Andy and Barney built a store on a vacant lot for Bert Miller (Sterling Holloway) in "The Merchant of Mayberry".