That's an interesting view.

If I were to fly to the USA, purchase something that was illegal in my home country (and explicitly state I was going to take it back home), then took it back home - would the vendor be prosecuted?

No because the sale would have had to occur in the place that it is illegal

If I fire off an agent to buy items at random times from a list which is updated in country, where does the purchase of the list items happen?

I'm sorry that you are not familiar with how laws work but it's kind of a big concept and I don't really have time now to teach it to you. I would recommend Wikipedia or perhaps ChatGPT to get started.

Law systems usually state explicitly their jurisdiction domain, depending on the level of offense.

If they came to your home country, very possibly yes.

? Of course they wouldn't.

Bartenders from other countries don't get locked up the moment they enter the US because they served alcohol to someone (a US citizen?) between 18 and 21. The US does not have jurisdiction over alcohol sales in other countries.

In this scenario, what's more likely to be illegal is bringing the item into the country.

It's difficult to make physical analogies to these types of internet laws. What makes them 'tricky' is how they are not physical.

If they pack the alcohol up in a crate, and then ship it to the person after they make the order in person? Less clear yes?

If the consumer goes to a place it is legal, and consumes it there without bringing any back, most people would say ‘meh’. Depending on the product. Hard drugs and sex work, being two common exceptions that some countries get more worked up about even traveling to ‘enjoy’ it.

But ship it back (especially hard drugs or sex workers!), and almost all people get more concerned.

The issue here is exactly why customs typically is a mandatory ‘gate’ for packages AND passengers entering a country.

Similar, one could say, to a giant country level firewall?

And why it is so lucrative for smugglers, which are defacto performing a type of arbitrage eh?

If you are purchasing any form of financial service that involves moving money around and said financial services provider also happens to interact with a US based financial entity, then yes, Uncle Sam will make life very difficult.

And no before you ask crypto won't solve this because Uncle Sam demands USD stablecoins to have sanctions mechanisms built in and clearing entities that don't implement KYC etc. will find themselves subjected to prosecution in other ways.

How many other laws can I passively break in other countries I have no connection to?

As many as you like, as long as you never travel to that country. Or that country has an extradition treaty with yours.