In addition; it would require quite a lot of labor to auction and send all these sets around, as well as paying for storage space. Something which is not accounted for in the 10-20% return figure.
Presumably no one bought the set, which is why it wasn't selling for very long. If you buy 1000s of sets you run the risk of Lego increasing production to compensate. Second you can't just sell all your sets at once. There might be demand for one set at $28. What about 2? 10? 100? So even with a 'winner' you may be only able to sell them in dribs and drabs, and even then only at an average of $14.
I’d think there may be an opportunity for a buying group or banner type consolidator though.
Or just straight up acquisitions, like independent pharmacies getting bought for 2-4x earnings, then sold as a package to public investors at 10x+ earnings.