A foreign country could arbitrarily decide I owed them a certain fine, or was no longer allowed into their country, or that they didn't want to allow my products into their country, at any time, whether or not I followed their laws.
In my daily life I've done, and continue to do, things that are illegal under e.g. Iranian law. That's fine and normal - I have no obligation to comply with Iranian law. Iran can make its own decisions about whether e.g. I'm allowed to enter their country, but that would always be the case.
If you sell electronics that are a fire hazard, you can be punished for breaking consumer laws. I mean, for an extreme example, if you sell an illegal substance in America from overseas you can be punished for breaking those laws too.
If I buy something mail-order from Canada, I'm considered the importer and would have to pay duty on it, just as if I had driven a truck over the border, bought the couch over there and driven it back.
If it's something as big as a couch, chances are it's going to be held at a customs warehouse for me to pick up (after I've paid the duty).
If I need to do this on a regular basis, I'm going to hire an import/export broker or possibly go through an actual furniture importer. That's the company that's doing business in the US that owes US incomes taxes, has to comply with US consumer protection laws and any of those other regulations.
In all of these scenarios, at no time did the Canadian couch store do any business in the US, even though I, the customer doing the "buying" may have been initiating the transaction over the Internet (or phone or with a paper mail-order form) physically in the US and/or with a US credit card.
If that Canadian couch is a fire hazard, the US's recourse is to stop it at the border and not let it in (or punish the US company, only in the case of the furniture reseller), and possibly punish me, the importer, since I'm the one legally attempting to bring it into the country. AFAIK, they have no recourse against the Canadian company.
If you decide to sell a couch to someone in America, you have to comply with American tax laws, American import and customs laws, American consumer laws, American patent laws, American copyright laws, American trademark laws, and any other laws involved with doing a financial transaction with someone in America. The same logic applies for Australia, the United Kingdom, Germany, Belgium, South Korea, Japan, etc. Pretending otherwise is naive, and if you don't believe me then try to sell something patented in America to an American.
The key question is what happens if you break those laws. In most cases you will be given a fine, and if you don't pay then you will no longer be allowed to sell goods to consumers in that country. If you continue to break the law then you are probably breaking an international treaty on border control or customs, which means that you could be extradited or tried in your own country. Some of the laws I mentioned above are mediated through international agreements, but the fundamental point is that if you break their laws they can place sanctions against you to stop you from doing business with them.
Of course, for a couch business things would probably never reach that level. And for an internet business you probably would just be IP blocked or something similar.
> No other country in the world can just make some "arbitrary" law that affects me. Unless my country agrees.
But it only affects you if you make the positive decision to do business with a country that has those laws. If you don't decide to do that, then you don't have to follow those laws (obviously). You can't have it both ways though (the benefit of having access to a market without having to follow the laws of that market).
In the case of enforcement you're right that they wouldn't have the right to compel to you to pay a fine, but they can in theory place sanctions against you. So if you continue to do business with sanctions in place then there is a process for extradition through international treaties.