@beeters The images shown are from sites where you can buy bundles, there, the legally acquired keys are usually displayed in this way. But there are also other possible applications, such as Keys from giveaways or those posted on Reddit or Discord etc.
Of course I can't guarantee that none of the keys people try to activate were stolen, but that doesn't matter, because this extension does absolutely nothing other than what you can do manually.
The sole purpose of this extension is to make activating keys faster and more convenient. And in order for this to work smoothly, the user must be logged in to the Steam website and the key to be activated must be valid.
I can therefore also rule out that someone will get into legal difficulties because of the use of this extension.