But if you sell your work you can buy more materials and make more work. Oh, hang on, writers only need a pencil stub and the back of an envelope. But painters ....
Anyway, whatever your artform, if you sell your work you don't have to have a dayjob and you can continue to make more work.
I'd say I get my validity from putting my work in front of an audience, from people viewing my work and reacting to it. Even if only one person sees and values one of my pieces and nobody else appreciates it, that is enough for me validity-wise, because that piece has found its audience.