How can I find out if a reseller site is Steam-verified?

[견과류 지배자 땃쥐]

and is humble-bundle and fanatical official reseller?

[Bee🐝]

Any site listed here is reputable - they get their keys from the publishers and developers: https://isthereanydeal.com/

But yes, Humble and Fanatical are great.

[Leonardo Da Pinchi]

I've used both Humblebundle and Fanatical for years, they're pretty reputable.

[견과류 지배자 땃쥐]

thank you so much

[Azure Fang]

There is no such thing as "Steam-verified" in terms of sellers. Sellers can be authorized retailers, but those are on a publisher-by-publisher and developer-by-developer basis.

As Bee posted, isthereanydeal is a great gauge of what is an authorized retailer, as the owner of the site is staunchly against grey market dealers. Additionally, pay attention to the sites themselves. If they say a game is "sold by" or "listed by" random companies or users, it's a grey market and not trustworthy.