I have an ipad from an estate that I am executor to. How can I reset the ipad so that I can sell it?

Here are the Apple support pages that outline the Activation Lock - and the only mechanisms by which it can be turned off:

https://support.apple.com/en-us/HT201365

https://support.apple.com/en-gb/HT201441

As you will discover, unless you are the owner of the AppleID that was last used to activate the device, or unless you can secure the cooperation of the previous owner, you’ll not be able to defeat or bypass the Activation Lock. Without documentary evidence that you are the original owner (such as sales receipt showing the serial number), Apple won’t assist - and nobody here will be able to help you.

What Apple will often do, with the appropriate paperwork, is to assist family (or possibly those will legal obligations) to recover access to the AppleID account - from which it is possible to remove the Activation Lock that otherwise inhibits reuse of the iPad.

How to request access to a deceased family member's Apple accounts - Apple Support

You may find this support article helpful with regard to deceased Apple product owner.

https://support.apple.com/en-us/HT208510