I'm in the process of selling my home and the buyer wants to keep all the automation. I've got maybe 60-80 Insteon dimmer switches, a handful of leak detectors, bunch of motion detectors, the ezflora sprinkler controller, mini remote, remote controlled curtains in home theater stage, thermostats, garage door sensors, etc, etc. Anyway, most of all my ISY setup and config was done years ago and I've forgotten lots of stuff.

I'm starting to worry about handing this complicated system over to the new owner and figuring out how to transfer the account, modules, subscription stuff over to the new owner. Is there by chance a guide somewhere out there that can be used for transferring ownership, and maybe a guide for new owner to get educated on their system (that is already setup for them)?

You may want to start by get some information in the UDI WIKI. https://wiki.universal-devices.com/index.php?title=Main_Page

There are some very good manuals, tutorials, setup videos  and the ISY Cookbook there.