How do I sell an iPad Air 1st gen?

You cannot sell it to Apple, but Apple may take in in for a trade in if it can be charged and powered up and will give you a gift card for it you can use to buy other Apple products.

click here ➜ Apple Trade In - Apple

Otherwise, you can put it up on Ebay or the like and clearly identify its not working or is inactive.

Don't expect a ton of money for an old device that is not working though.

The original iPad Air was released by Apple 10½ years ago. I haven’t been able to look it up with no serial number (however, you can at Apple Trade In - Apple), but I think it’s safe to say Apple would offer you no trade-in value for such an old device, but would offer to recycle it for you.