I've been buying things online mostly from Japan for a little over 10 years now. I have way too much stuff but often have difficulty selling it because of my eclectic taste and collector's autism.
>Mercari
This site is somewhat recent but you can find alot of gems here, especially when someone lists something for super cheap when they don't know the proper value of it due to age, scarcity etc. Being used primarily by individuals rather than stores, people sometimes fail to have good photos, tagging, descriptions, measurements etc. If you use a good proxy you can haggle with sellers.
>Yahoo
Used to be good but most individual sellers moved to Mercari. Mostly reuse shops now just listing buyout prices, but the occasional individual seller. There are a few auctions still mostly from individuals, Book Off or Realclothes.
>Rakuten
Encompasses most of the below stores as well as some others. Contrasts Mercari and Yahoo with a great tagging system that allows you to search by material, pattern, colour etc.
>Bring and Rinkan
A little too focused on high street but you can find great bargains on other things outside of those brands.
>Kindal
Great variety and usually great prices. Some things are a bit overpriced but quite good overall. The Americamura one and one of the Nagoya stores are the best reuse shops I've been to in person.
>Brooch
Similar to the above but a smaller selection. Typically focused on high end Japanese designers.
>2nd Street
Japan's largest chain of reuse shops handling everything from figures to fine china to whiteware to camping equipment. The online store has tonnes and tonnes of clothing coming in every day. They typically don't have great photos of items unless they're taken to highlight damage, a stain etc and sometimes the descriptions are lacking. The condition ranking can sometimes be misleading; I've purchased a C condition item that was in flawless condition but also seen B condition items that had severe colour fading.
>Ragtag
Sometimes okay and you can sometimes find a bargain but I've typically found them to have worse photos, worse variety and higher prices than the above. In particular they rarely disclose product codes so it's near impossible to see what an item looks like outside of the photos they've supplied.
>Treasure Factory (TreFac)
Similar to 2nd street with more of a focus on cheap vintage clothing. The sort of store you have to dig through to find a gem, but it can be worth it.
>Grailed
Grailed used to be good before they launched out of Beta and started aggressively focusing on making money. I get it, it's a business, but the quality of the site really started to go downhill as they tried to onboard as many people as possible. The price of everything went up, as did their cuts, and now they collect sales tax for your local government, meaning you have to pay an extra 10 or 15% or whatever on top of whatever the item + shipping totals to. Selling on there is also a nightmare because the majority of buyers are <25 and lowbally. I also believe, based on both what I have read from people in the know and my own observations, that Grailed harmed internet fashion as a whole both in terms of homogenization, drying up of availability of items in Japan and increasing prices for used designer clothing across the internet.