where you have to trust that the cash the buyer is giving you is not counterfeit

Blockchain doesn’t solve that.

or if using a bank transfer, that they will actually make the transfer once you signed away your ownership

Blockchain doesn’t solve that.

The blockchain would be the ownership database maintained by the country

Why would you use blockchain instead of just a database. Blockchain is an extremely expensive, inefficient “database” that only makes sense if you don’t have a central authority you can trust to run the database. If you’re relying on the country to run and own the database, blockchain offers nothing useful.

In case you don’t know what I mean by “atomic”, I mean that the two operations cannot be done separately

Yes, it’s a very standard property that most databases have, along with consistency, isolation and durability. So-called ACID. Again, this isn’t some magical “blockchain” thing, this is a basic database feature.