From Guinness Book of World Records 2010 edition:

Led Zeppelin broke the world record for the Highest Demand for Tickets for One Music Concert when 20 million requests came through for the one-time reunion show in December 2007.

Led Zeppelin’s (UK) one-off reunion show at London’s O2 Arena on 10 December 2007 attracted more than 20 million ticket requests. Such was the demand for the 9,000 pairs of tickets that were made available to the public that they were reported to be selling for up to £1,800 ($3,654) on eBay – more than 14 times their original price of £125 ($253). The two-hour concert, the legendary rock band’s first full show since 1979, was dedicated to Atlantic Records founder Ahmet Ertegun (USA, b. Turkey, 1923–2006), who signed the group in 1968.