People do sell loot box items in a lot of games... that often just makes the gambling problem worse, because people buy the loot boxes thinking they will make real world money.
Good timing for this - the Team Fortress 2 hat market just collapsed over a change in drop rates.
It's probably worth differentiating tradeable loot boxes, which end up used like stock speculation, from untradeable ones, which seem to be used more like slot machines.