It doesn't matter why. Just pick one from each group:

More than 250m albums sold:

The Beatles
Elvis Presley
Michael Jackson
Madonna
Elton John
Led Zeppelin
Pink Floyd

200 to 250m albums sold:

Mariah Carey
Celine Dion
AC/DC
Whitney Houston
Queen
The Rolling Stones
ABBA

120 to 200m albums sold:

Rihanna
Taylor Swift
Eminem
Garth Brooks
Eagles
U2
Billy Joel
Phil Collins
Aerosmith
Barbra Streisand
Genesis
Neil Diamond
Kanye West
Bruce Springsteen
Bee Gees
Julio Iglesias

100 to 100m albums sold:

Metallica
Jay-Z
Bon Jovi
Britney Spears
Rod Stewart
Fleetwood Mac
Backstreet Boys
Guns N' Roses
Prince
Paul McCartney
Kenny Rogers
Janet Jackson
Chicago
The Carpenters
George Michael
Bryan Adams
Def Leppard
Cher
Lionel Richie
Olivia Newton-John
Stevie Wonder
Linda Ronstadt
Tina Turner
Donna Summer
The Beach Boys
The Who
Barry White

80 to 100m albums sold:

Katy Perry
Santana
Earth, Wind & Fire
Lady Gaga
Shania Twain
R.E.M.
Bruno Mars
Van Halen
Red Hot Chili Peppers
Foreigner
The Doors
Reba McEntire
Meat Loaf
Barry Manilow
Tom Petty
Johnny Hallyday

75 to 80m albums sold:

The Black Eyed Peas
Beyoncé
Journey
Kenny G
Enya
Robbie Williams
Alabama
Green Day
Tupac Shakur
Nirvana
The Police
Bob Marley
Kiss ]
Depeche Mode
Aretha Franklin