Book early

As the BBC prepares to launch the Proms 2016, I'm slightly puzzled as to how two major ticket resellers already have tickets on offer. General sale only opens on May 7, yet Viagogo have seats available for the first night at £265.96, and the Last Night at £819.90. Stubhub has the first night at a mere £182.00, and the Last Night at £1298.00.