There is no way at all to verify they are not selling your data. Also, there is no law that forbids them to do so.
Also, why would they sell your data? The bank accounts you connect to YNAB already have your data and sell it. Do you also not use banks?
Where i live (Netherlands, Europe) banks are not allowed to sell our data.
I also don't use Facebook, and try not to associate myself with their drama
That's what they say (just as Ynab says.) But that's not what really happens:
"How to Check If Your Facebook Data Was Sold Off": https://www.makeuseof.com/tag/check-facebook-data-sold-off/
Also, why would they sell your data? The bank accounts you connect to YNAB already have your data and sell it. Do you also not use banks?