I'd suspect that Clarke's Odyssey series would be up there, too. No prizes for guessing the top-selling series in fantasy (per book, at least); Potter must be right up there, with - if memory serves - something like 400 million books sold in total. Nearly 70 million per book. Yikes...
You're forgetting
Lord of the Rings: 250 million + copies sold of the single book. Even if you want to argue it's three books (it isn't, it was merely split due to size) that's still 83 million apiece.
The sales for the fantasy top sellers seem easier to track down:
JK Rowling: 350-400 million.
JRR Tolkien: 320 million (including
The Hobbit and his other books).
Stephen King: 300 million.
Terry Pratchett: 50 million.
Robert Jordan: 35 million.
Terry Brooks, Raymond E. Feist & David Eddings: 20 million each or thereabouts; Brooks is probably the biggest-selling of those three.
Margaret Weis & Tracy Hickman: 20 million (combined sales of their jointly-written novels).
Terry Goodkind: 15 million.
George RR Martin: 5-7 million.
David Gemmell: 2-3 million.
Steven Erikson: Probably around the 1 million mark, maybe more.