There are a few countries where scam postings frequently originate, including expensive camera gear. Japan isn't one of them. Scam websites exist right around here too, in the SF Bay Area. There are crooks everywhere. Legit dealers work very hard to protect their reputations. EBay has a list of common sense guidelines for spotting and reporting suspicious listings. Retail dealers, unless relatively new, have a track record
of repeat customers. But beware of on-line "ratings" or rating services, which can actually be purchased or edited for a fee, or skewed the other way by the occasional crank complaint. These forums tend to be a pretty good avenue for asking around.