Yeah, the containers are just for storing it. You have to take it to one of the points on the map to sell it (press the "9" key to see the map and look for the trailer icons, but not everywhere will accept every crop harvested). On the Bjornholm map the Windmill and Grain Elevator seem to offer good prices for the basic crops (wheat, barley and canola (oilseed rape over here)) on my save.
The idea behind storing goods are to wait until the prices are good. To keep track of prices press the "I" key to get to the inventory screen then press the upper-right arrow once. To scroll through the prices click the arrow on the middle-right.
Sometimes you'll get "great demand" prices, where a sell-point will offer extra for a product, so if you don't need the money immediately it can be worth it to wait until the prices are good or in demand. You'll get a quick notification on the screen if a crop is in great demand.
Keep in mind that while oilseed rape sells for more, you get less yield from a field, so it's not necessarily the most profitable. Also make sure you fertilise your fields (fertiliser, sprayer or manure) as it doubles the yield, and will keep you much busier with your trailers.