Is there a way to export a master list of all magic cards from all sets in a csv file?

Not without some legwork, but it would be possible with a premium subscription. As a premium member, you have the ability to add 1 of each card from a given set by going to that set's page. So you'd have to

1. sign up for premium (there's a 7-day free trial if you haven't used it yet)
2. open an edition (I'd recommend opening each edition in a new tab (Ctrl+click the link) then closing when done with it, as the main edition page can sometimes take a few seconds to load all the editions, and then you have to find your place again)
3. use the "add 1 of each card" tool
4. repeat steps 2-3 for every single edition
5. export your Inventory to CSV as normal

I know that's not ideal, but at least it's possible.

Unless you really need Deckbox's particular listings, maybe MTGJson is what you want to use?