"The artist sells his sculpture."
Translation:Der Künstler verkauft seine Plastik.
"Plastik" is a valid word in this context. see here: http://www.duden.de/rechtschreibung/Plastik_Skulptur_Anschaulichkeit
But I would more likely use other words anyways.
Like, well, "Skulptur"...., or "Figur" or more general "Werk"
"Plastik" is a kind of material. There are artists who make sculptures from wood or metal, or stone. Are they all called "Plastik"? The english word: "sculpture", comes from the verb: "to sculp", something like "basteln". But what is the german name of the product of "basteln"? Eine Plastik?
I've read all comments. Even though it is correct connected with art, it is unusual for a normal German to say it in this way. The majority of Germans would say 'Skulptur' instead of 'Plastik'. If you tell that sentence to a normal German, who probably didn't study art, he will in normal case think of normal plastic (Plastik) instead of a sculpture. So even though it is correct, it's unwise to use it while talking to normal people, who are not affiliated with art.