The difference between a single node backed by PostgreSQL and OpenSea offering an API to interact with the blockchain is that in theory you could interact with the blockchain yourself. It's something like Wikipedia. I could run my own local cached copy by using the periodic wiki dumps. But its easier to go through the live Wikipedia.

Are there SQL servers intended for direct public usage? EDIT: I do understand the ubiquity of SQL servers :) I'm asking about ones that expose a good chunk of SQL to end users, since theoretically one could build on that

What’s the point of this ask? A blockchain’s dataset isn’t arbitrarily editable by its users — there’s a very specific set of actions you can do, which are well defined, that modify the chain in a very structured manner… just as any API sitting in front of a database would do.

Obviously you can modify your local copy of the chain as you wish, even corrupting the data, but if your chain isn’t accepted by the public, your changes don’t mean shit. So you’re really only able to (meaningfully) execute those public-allowed operations

REST APIs usually have a database behind them, yes.

That's not what I meant, I meant 'direct' as in permissioned by end users

NFTs aren’t permissioned by end users. The person that deployed the smart contract still controls all permissions (although they can’t change it later — that is the only difference).

The reason that you can trade a NFT is because the smart contract has an API written by the creator to allow it.