Steve H brought next idea :
Well, I fix my things up for my own benefit and use, often for the entertainment and learning value. The object is never to resell them because there would (usually) be no profit in doing that, once my labour is taken into account. At the end of the shows, they cost everything up, then call the difference between buying + parts and selling, the profit margin. They never mention the very expensive cost of labour.
They make it not appear to be a hobby, but rather a commercial enterprise - an expert operating in an expensive well equipped workshop working long hours on a project.