This is where Avi Arad gets some rather unfair blame. The Fantastic Four film rights had nothing to do with Avi Arad. 20th Century Fox acquired the rights to Fantastic Four (and X-Men as well) back in 1994, back when Arad was a part of ToyBiz. Arad became more involved with Marvel in 1995 when Ike Perlmutter started his bid to gain full control of Marvel and Arad's involvement with the movies started in 1996 when Marvel formed Marvel "Studios."
The fire sale that happened in 1999 involved Sony's Columbia Pictures getting the rights to Spider-Man and Daredevil for dirt cheap. But it's not like Marvel was in a position to demand more money. Arad was able to get more money later on for future rights sales like selling Daredevil to New Regency/20th Century Fox and Thor to Columbia Pictures, when Marvel was boosted by the success of Spider-Man and X-Men.
Before Arad was involved with Marvel, Marvel sold the film rights to their properties for absurdly low prices. Like pathetically low. Marvel sold the Fantastic Four film rights in the 1980's to Constantin Film for $250,000. Spider-Man was sold to Canon Films for $225,000.
If anything Arad helped them quite a bit by getting Marvel off the ground to where they are today. While he deserves a lot of criticism for the way he runs the Spider-Man film series, Arad doesn't deserve a lot of the hate he currently gets.