They usually go back further than 30 days. I don't know how it's supposed to work but sometimes they go back months and other times not.

I can think of other ways to check how often a seller has sold that item but I'm not going to say how. I have someone who watches one of my items almost permanently so that they can check my sales and I get a bit annoyed that I have no privacy. I understand that we need to be able to do some research on prices so we know how much to ask, but I honestly don't think it's right that other sellers can see so much of my business, and so easily. It's like a b&m shop putting their sales records on public display!