This. Plus different sellers have different motivations and different pricing strategies. Stubhub based on their volume has a sense of where the market is, and if they see a seller underpricing versus what Stubhub thinks the ticket will sell for, they may swoop in and try to capture some of the upside.

There's no such thing as a ticker feed for the resale markets -- the market is characterized by "imperfect information". Artists and venues making it hard to buy and sell basically widens the spread for the secondaries to play with.