A lot of grain was sold "in the car" but it was not at all like roller lumber, where circuitous routing was used to lengthen the time of the trip to permit  extra time to market the load.  In the case of grain there were two principal ways for the shipper to bill the grain "Waive Inspection Set Direct" or "Allow Inspection" .  Allow Inspection cars were held in "grain yards" where the cars would be sampled and the grain sold on the trading floor of whatever Grain Exchange or Board of Trade governed.   Once sold updated billing would be provided to the railroad.   Minneapolis was the largest cash grain market in the world and most the railroads has these grain holding yards on the west side of town.including:

Milwaukee Road -- Bass Lake Yard

Great Northern - Cedar Lake Yard (gravity hump with track skates)

Northern Pacific - Grove Yard

Soo Line - Himboldt Yard (still in use for general traffic)