It's certainly possible to open a US Amazon Seller Account from anywhere although there are some caveats to that.
We are based in the UK and have accounts setup on multiple Amazon marketplaces (including Amazon USA) and we have seen people from over 100+ countries get started on Amazon.
If you are on the list of approved countries Amazon accepts, then it is very simple and just a case of signing up and logging in.
If your country is not on the list of approved countries then it is usually still possible to open an account with a few extra steps. This varies by country, but usually just involves getting a US address through a mailbox service such as EarthClassMail, and a US bank account through a service such as WorldFirst. You may also need a chargeable credit card, but you can sometimes use your own depending which country you are in.
We are working on putting together some simple training videos to walk people through this, but we don't have a timeline when this will be ready yet.