Retail Websites:
The point of this article is so educate, not shame. I run into a lot of people that purchase, or almost purchase, recasts unknowingly. I also have potential clients that want a second opinion if they’ve purchased an original kit. It is now easier than ever for overseas hobbyists to support circles by buying directly. In my opinion, if you want this hobby to flourish, we need to support the people putting in the work and mastering their craft. If you still choose to purchase recasts or unlicensed kits after reading this article, that does not concern me. I can’t control what you do, I just won’t accept your commission.
The websites listed below are the sites I am familiar with. I have included general guidelines for spotting unofficial websites and recasts. Please have caution when purchasing from unfamiliar/suspicious websites. If you are still unsure if a kit is authentic you can email me or contact me on social media.

Booth.pm is a only sells authentic kits directly from the circle/sculptor. All kits sold here are authentic and have permission from the copyright holder.

TFO Hobima is a bi-yearly online hobby event. You must have a Japanese address to register an account and purchase a ticket to “attend”. All kits sold at TFO are originals directly from the seller with permission from the copyright holder.

orzGK technically sells original kits, but they are not licensed. Kits of copyrighted characters are not made with the permission of the copyright holder. Often they make figures and kits based on fanart with permission (eventually) from the artist, but they are still unlicensed. There have also been controversies involving stolen photos to promote their products.
If there is an original character kit on their website that interests you, I would highly reccomend trying to buy it on other sites like Booth or Taobao.

E2046 is a recast company based in Hong Kong. A majority of their products are copies of kits and figures with no permission from the original artists/sculptors/circles/manufactures/etc. Recasts are technically bootlegs. All they need is to acquire one copy of a kit in order to produce hundreds of copies. The Anti-hero line is recasts of PVC figures into resin. The Gathering line is pre-painted kits made in-house. The Ori line of figures and kits. Ori figures are made by their in-house sculptors, but are still unlicensed.
Compare the cost of an original copy of this Angel kit by Yakiimo vs their recast:

GK-Model is a recast company like E2046. Every kit they sell (except for their Original League) are recasts. None of their profits go back to the original sculptors and circles that make the kits in the first place. Their Original League produces kits sculpted by artists with permission, but they are still unlicensed.
Secondhand Websites
Toshki has a in-depth guide on her blog with more details about spotting recasts. I tried to write a quick guide of my own, but realized hers was way better.
Still Unsure?
Reach out to Kalmia via email, during a stream, on social media, or on her Discord channel (there’s a bunch of other Gk information there too)