The Apple AirTags are designed to be used to track your stuff, you can attach one to your keys, your bag, or put it in your wallet. As the device can track things, it could also be used by people to track other people without their knowledge.
Apple has two security things built into the AirTags to stop this, one is by playing a sound if the AirTag remains separated from the iPhone that it is paired with. This is designed to let people know that there is an AirTag close to them.
The second security feature is to display a message on an Apple device to let them know that there is an Apple AirTag nearby them. There is also an android app that users can install to see if there are any AirTags near them.
Apple has added these two features in an attempt to stop people from using them to stalk people, someone on Etsy has decided it would be a good idea to disable the sound on the device and then sell them for a profit, what this basically does is make the AirTag harder to detect, this can be seen in the photo above. There are apparently some of these devices also being sold on eBay as well.