https://wccftech.com/how-to/how-to-remove-airtag-from-your-apple-id-account-before-selling-it/

An AirTag is a very useful accessory that you can attach to your belonging in order to track their location. Announced a few days back at Apple's Soring Loaded event, the AirTag is priced at $29. In addition, you have the option available to buy a four-pack for $99 which will save you some extra bucks. If you happen to have a lot of AirTags at your disposal and want to sell them, you should remove them from your account before doing so. If you are not familiar with the method, follow the step-by-step instructions below on how to remove an AirTag from your Apple ID account.

If you have decided to sell your Air Tag or just lend it to your friends or siblings, you should first remove it from your Apple ID account. If you do not do so, the AirTag will continue to report its location on your account. The process to remove AirTag from your Apple ID account is fairly simple and you do not require the use of a computer whatsoever. All you need to do is find the right settings and you are good to go.