It amazes me. I have no understanding why Japanese sellers are so proficient. I have had so many utterly awful experiences with online sales and even shops over the years that I head straight to Japan for any second hand gear.
I think of lot of it is culture based (on many levels). Generally speaking, people prefer new and perfectly working items - when things break down or look like crap they tend to become garbage, rather than reused or seen as a way to make some money. This is not just a camera thing. When I first came to Japan (1999) it was easy to furnish your apartment by sussing out the big
gomi (trash) days - people would throw out perfectly working TVs, computers, refrigerators, sofas, etc. because they wanted something newer, bigger, or better, and since there was nothing they could do with the old thing, it just got junked. (That doesn't happen anymore - we have to pay for large items to be taken away/recycled.) Of course, people don't do that with cameras/lenses, which often get sent to the second-hand sellers. Even then, they are still in good condition. A lot of places, including shops, have limited storage which means there are no "attic" finds or estate sales where cameras and lenses that have been sitting unused for decades will show up. I think most people (men) who deal with used cameras and lenses are focused solely on that enterprise, and know that they can get a better sale is the item is in good working and cosmetic order. I have seen the repair corner in some shops so I guess quite a few do in-house repairs. People are pretty honest here and do the right thing almost all of the time - so selling "junk" listed as something better isn't as prevalent as it is with other sellers from other countries. I'm currently sitting at Starbucks and I can reserve my table by placing my wallet or phone on it, and I often leave my laptop on the table when I use the restroom. I would never do that anywhere else, but I know that people here will not touch it because it's not theirs. In fact, I'm so used to people doing the right thing here, I have to remind myself to be more alert when I travel or go back home to Canada.
That said, I don't want to imply that sellers (or Japanese in general) are perfect -- it's far from ideal here and there are a lot of areas that the Japanese seriously need to work on. The other thing I don't like about how well-regarded used film and darkroom equipment is here is that it can often be overpriced due to overvaluation. However, over the years I've seen camera/lens prices get better (maybe because fewer people are buying them?) but I had a hard time finding an enlarger here because people wanted too much money for basic used models (over $1000). Luckily I found somebody giving one away. Now I just need to find someone giving away a 4x5 enlarger and I'll be set!
FWIW, I prefer to buy most of my photo gear from Japanese sellers as well - I trust them and I don't have to deal with expensive shipping or currency issues.