NIKON AF-S Nikkor 12-24mm f 1:4G DX Lens

I have been looking around for a wide angle lens for my D7100. This lens recently came up for sale by a private individual for around $500. Good / bad deal? What do you know about this lens and how it would perform on my d7100?

I'm trying to look up the lens and all I can find is 12-24 f4. Don't know why I can't find a F 1.4?

many thanks.