buffy.nu is just a site who picks/steals info from other sites and repost them ( a lot without permission)
Sabrina the teenage witch is not really something that is respected (no disrespect to people who liked the show and the movies). It would be a huge step back in her career and that's why (and the unreliable source) I don't believe that she really would do this. And it also doesn't make sense.
If there's a movie that Sarah really signed for, it will be posted at Variety.com. Variety is a very reliable source and they were both the first one who posted the news about Sarah signing for
Southland tale and
The Grudge
ETA: Buffy.nu is a buffy site made by Buffy fans. They get there information by reading other sites and post them. Even if it's tabloid news.
Buffy.nu says:
Sarah Michelle Gellar - ’Buffy’ star gets ’Sabrina’ movie role ?
From Teenhollywood.com - 2004-09-18th
And teen magazine picked this from a UK tabloid magazine.