How can I access contact info of Amazon sellers on US & UK marketplaces?

You can contact Amazon sellers individually by clicking on their business name and then on the "ask a question" button, after which you'll be able to contact the seller via email.  Some sellers will leave a phone number to contact them directly. On the other hand, gathering a directory, or records, of Amazon seller contact information, is trickier to do, and one needs to be careful not to break Amazon's Terms and Conditions.   You could try one of the email database marketing services available online.  The question you need to ask yourself is, what do you hope to achieve with this information: is it simply to contact the seller? you can do that easily through Amazon.  If it's to ask a specific question about a product, or, alternatively, find out how he's successful in his Amazon selling business, you can extract that info, too, without leaving Amazon.  You need to pay attention to his product's presentation, description, customer reviews, shipping terms, how he handles negative reviews, etc.  Remember that circumstances might change for different Amazon marketplaces, and Algopix can extract for you in-depth product research, as well as competitive analysis.