What are trustworthy key shops?

Humble Bundle is trust worthy. if you have to send them message because of key issues then never mention Steamgifts if you plan on giving away games here. They don't like that :P

If you aren't sure, steam itself is always good place where buy games, especially in sales.

That site you're using is pretty good at showing you that info.

If you're worried about trusting a site, you can assume that any store not in the "Official stores" section is shady to some degree. That doesn't mean you won't be able to make a purchase, but it means that there's something off. It could for example be that they don't disclose where they got their keys from, try to upsell you on "premium subscriptions", or that they resell keys from users. That site you're using does list the individual risks for each such site though, just click "Risks" on the keystores.

Something like Fanatical is an official store though. They get their keys directly from the publishers themselves and it's no different from buying from any other official store like Steam.

All the sites that come up under official stores on ggdeals are pretty trustworthy.

Fanatical and Humble Bundle are the only ones that I have used but I have heard greenmangaming is a good one too. Indiegala is another one.
You can also check isthereanydeal.com out. afaik, they list only official/good key stores.
A friend of mine uses grey market keys a lot, so if you want, I can ask them. Though grey market will never be risk free.

As you're using ggdeals you'll notice they discern between official stores which source their keys directly from developers/publishers and keyshops where the origin of the keys is shrouded in mystery and the shop is only functioning as a platform for key sellers like an ebay for keys.
The inclusion of the so-called grey market keyshops is the reason why linking to ggdeals is against SG rules.

Fanatical is a trusted site although they have adopted the slimy practice of mystery bundles to sell off their shelf warmers.

Most of the keyshops on ggdeals.com are legit, but please for the love of god and your sake, don't use the following: CDKeys, G2A, Kinguin. They are sketchy as hell and their support is terrible, even with buyer protection options.

Use ggdeals to check shops
Here is awful rule u cant mention keyshops for some stupid reason...

Also, in the keyshops list of gg.deals, you'll see for each listing a "# risks" reference: "1 risk" "5 risks"

If you click on those numbers, you get a pop-out message where gg.deals tells you the specific risks of using that shop.

I typically use Fanatical and Humble for bundles, and Green Man Gaming for individual games. These have been my go-to stores for many years and likely will be for many more.

I also use gg.deals a lot, just with sketchy key shops turned off so I never even have to see mention of them.