"The artist sells his sculpture."

[Rakascha]

"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"

[avaschmys]

I've never heard "Plastik" used for "sculpture". Is this a mistake of DL?

[az_p]

No, it's correct.

[FreekVerkerk]

"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?

[Ray493065]

shouldn't seine be sein (since it is akkusativ and is modifying neuter)

[Muhammad704143]

I think it would be better to specify his work (artistic sculpture) rather than saying what it's made of.