I've had mixed results with Japanese eBay sellers, but it's definitely true that they dramatically overrate their gear. If it says "Exc++++", that means fair. If it says "Near mint" that means modestly above average for its age, maybe. It's not the worst problem in the world; you just need to apply a correction factor to the descriptive rating, as you would with a neutral density filter. Judged on its own terms, it's just a different scale, like a guitar amp that goes to 11.
However, twice recently I've received products from Japan with undisclosed defects. The first time, the seller readily refunded part of the purchase price and that was a satisfactory solution. The second one I didn't discover until several months later (I made the mistake of not testing it right away), and that has turned out to be a bad investment on my part (about $250, so not the end of the world, but annoying). Of course I can't prove it, but I suspect the seller's plan was to refund part of my money if I noticed, but only if I noticed.
I'm much more wary now. To be scrupulously fair, I've also received items with undisclosed defects from U.S. and Canadian sellers, including a Canadian camera store with a great reputation (I'm sure they did not know they were selling a defective lens; I didn't figure it out until I'd been using it for a year).
That tactic of selling something that has a defect and hoping the buyer doesn't notice, with a backup plan of accepting a return or issuing a partial refund, seems to be par for the course on eBay these days. (It was not common when I first started buying and selling on eBay far too many moons ago.) And just to reiterate, I think this applies to sellers of all kinds and places, from professional eBay operations to individuals clearing out their closets. It's part of the online culture now.
Anyway, bottom line on my meanderings here is that you should thoroughly test anything you buy on eBay, and do so soon after you receive it.