Usually I buy on Yahoo Japan and not Ebay, but I decided jump on the band wagon just for fun. I offered around 1100 Yen for Lemmings when his price on Ebay was about 2200-2100 Yen. The price I offered is exactly what I would pay BIN for the game on Yahoo and he rejected it.
I did not link him to the Yahoo Japan seller out of politeness, but I could tell that he's not exactly aware of his pricing. However, if you go slightly under his ask ($4-$6 about), he'll likely accept. The big thing is, he rarely communicates to you or follows up on things if something happens, so if your looking for 'service' you probably won't get it.
You definitely can get better deals if you know a bit of Japanese. When I was bargaining with him, I always provided my offers in Yen (with dollar value) just to politely remind him what price he's choosing to accept or reject.