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.
