What are by tax obligations in the USA?

There are two main considerations. First, if you are selling products to U.S. residents located in various states, you may be liable to collect state sales tax depending upon the residency of your customers. Second, you may be liable for federal income taxes on your net earnings if it's determined that you are engaged in a U.S. trade or business and have income effectively connected with that U.S. trade or business. The rules for determining whether U.S. source income is effectively connected income are quite complex with many nuances. For the sale of inventory in the U.S., the old "title passage rule" used to apply. If title to goods passes outside of the U.S., income from sale of the goods was deemed foreign source and not subject to U.S. federal taxes. The recent tax law changes under the TCJA have modified these sourcing rules. If products are manufactured outside of the U.S. and then sold to U.S. customers, the proceeds are foreign source and not subject to U.S. tax. You also need to be mindful of the treaty provisions that may apply. The U.S. and U.K. have an income tax treaty which provides that a U.K. company's business profits will not be subject to tax in the U.S., unless the U.K. company operates a permanent establishment in the U.S. What constitutes a permanent establishment depends upon the specific facts and circumstances of your situation, so it's important to consult with a knowledgeable tax advisor and consider all of these issues.