As a U.S. citizen, you always need to be mindful of your U.S. tax filing obligations, regardless of where you live or where you earn income. U.S. citizens are subject to income taxes on their worldwide income. If you live abroad as an expat, you may be eligible to take a foreign earned income exclusion for wages paid to you whilst you are physically present outside of the U.S. If you are looking to run an Amazon FBA business, you need to be mindful of the U.S. federal income tax obligations, as well as the state sales tax obligations that may arise depending upon your sales volume in each U.S. state. I would recommend consulting with an international tax CPA or tax attorney that can advise you on these various tax issues.