The amount of Yankees tickets available for auction are obviously much greater than the amount of Mets tickets for two reasons:

1) Yankees fans are non-loyal fair weather winner pickers who sell their tickets like they are a plummeting stock

2) It's well known that corporations own more than a fair share of season tickets at Yankee Stadium. I mean - why let your fans have decent seats or tickets to big games. With corporation after corporation reducing costs and cutting corners, it's no wonder that they are unloading their tickets on ebay for some breathing room in their P&L.