The “Sell” button in your Barn being disabled during Competitions is somewhat of a coincidence, or only part of the reason. If we had to start the game from scratch, we would remove the button and replace it with options to sell your produce such as the Train, Cheap Train, Orders, plus other Barn management tools.
Also we plan other fun ways of selling besides the Train and Orders (Truck Deliveries).
We believe selling from the barn is just bad gameplay — as proof, many, many players didn’t bother to sell and just accumulated lots and lots of things in their barns.
Most probably, we will remove the “Sell” option from your barn, as you can instantly sell with your Cheap Train (if you really need some quick coins to plant), or the regular Train.
As a side note, selling from the Barn is also not realistic, because in real life you don’t instantly sell what you want when you want to — or at least not for full price, anyway.