IPad storage

The internal storage of your iPad cannot be changed or updated. The amount of internal stored is fixed for the life of the device.

Whilst you cannot increase your local storage, you can subscribe to additional iCloud storage space - but this is not a substitute for internal storage on your iPad.

About storage on your device and in iCloud – Apple Support



I don’t think you could unless it was within a short time frame as they would basically say that you chose the wrong one. They would lose too much money on it as they now can’t sell yours as a new iPad. You could just look at the trade-in value with Apple to see how close to the original value you could get, but I think you would get more money selling it privately