Check out our Yahoo Japan search guide here: www.elitefourum.com/t/yahoo-japan-search-guide/13488/1 This will help with navigating and finding what you want.
There really aren’t any super secret special deal tips. Just check frequently, make sure you know what you’re buying, and avoid common scams like lottery listings and vaguely described bulk lots with a few cool cards on top (westerner bait).