Moby Games only lets you view a page of 25 entries at a time, so that would be extremely lengthy even if automated.
Searching around, I discovered that at least Steam has an API to access, including the ability to retrieve a listing of titles purchased in a user's account:https://developer.valvesoftware.com/wik ... 28v0001.29
GoG has promised to open source their Galaxy browser, but that's whenever they get to it.
I haven't touched UPlay or Origin, so I really can't help you there.
As for Amazon, I poked through the account management to see if there was some way to export a list from their site, but I didn't come across anything.
Apologies for my not being too helpful, but hopefully this gives you some ideas.