Take a look at the stars who have been traded in the past 10-15 years and tell me how many deals have involved a young player clearly better than Sullinger.
I might take you up on your homework assignment later, but we've already mentioned Al Jefferson. The Lakers got young Caron Butler and young Lamar Odom for Shaq; those are both upside players who peaked at a fringe all-star level.
Keep in mind we're not talking about eventual value, we're talking about perceived value at the time of the trade. A player who was seen as more valuable but ended up sucking counts against Sullinger, a player who wasn't seen as more valuable than Sullinger but peaked higher than you expected Sullinger to peak doesn't count.
Unless those are the only deals ever made for stars, it seems likely that a decent percentage of such trades involved packages where no player was clearly better than Sullinger.
I'll help you out a bit. Here are (most if not all of) the players named to the first, second, or third team All-NBA in the past three seasons who have been traded (not including sign-and-trades) at some point in their career after becoming established players (so not Marc Gasol): Chris Paul, Carmelo Anthony, Dwight Howard, James Harden, Pau Gasol, Zach Randolph
Is a package of Sullinger, a non-ridiculous number of draft picks, and matching salary of equal or greater value to some of the packages used to acquire these some of these players?