Violin questions from a beginner?

I've been extremely interested in playing the violin for a long time now, but I have some questions. I know about the sizes, but is it judged just on arm length. Also I've seen some violins on eBay and they're only about $50 which sounds very iffy to me. At my towns local music store they're about $1000 which is way too much for me to spend. So I'm wondering, where I can get a cheapish violin for a beginner, but not get completely ripped off?