There is no single answer to the question. It all depends on the show/fair. Generally market birds are intended to be sold, but the fair may have limits on the number of birds that can sell, or how they much place to be sold. Exhibition birds are typically not required to be sold, and it may not even be available as a part of the fair or show.
You need to contact the Show Superintendent and ask specifically for that show.
hey i was wondering, i have two baby silkeys at work, there 4 weeks old, but how old do they need to be before entering them into any show? and whats that about selling them after a show? this would just be a bit of fun