We hadn't noticed this before, but seems that if you do a search for an item on Yahoo! Japan Auctions, a link to an RSS feed for that search appears at the bottom of the page. We much prefer this to the email alerts that were previously the only option. There don't appear to be any feeds for categories, presumably because the number of items would max out your newsreader in very short order
http://www.dottocomu.com/b/archives/cat_japan_news.html