I suspect the game has some kind of value it uses for all items, including stuff like piggybanks and books which can't be sold.
The Shabati sculpture has a value. They sell so rarely it shows NA, but if you click on the little chart icon, so you can see the history, you can see values on that chart.
I suspect it uses the most recent value. But I'm guessing, I don't know that.
I do know how you can test, at least with it.
Modifiy the link below to use your player ID in place of 12345, and your API key at the end.
http://api.torn.com/user/123456?selections=networth&key=Your_API_KeySave that URL. Then using that, you can get your exact, up to date networth.
Then send the Shabti sculpture to a friend and you can see how your networth changes. If they are a good friend, they'll send it back. If not, then it's a flip of the coin.