how to change the currency to U.S. dollars on online sites from USA?

Every time I use an online shopping site, the prices are shown in United Kingdom's currency of Pounds sterling, not in U.S. dollars. NONE of the URLs for the sites indicate that they are in England (showing in the URL the extension). (I have bought things on ebay and amazon.com US locations from European sellers in the past and so at some point the prices were correctly shown as pounds. But HOW do I change the default for Safari so that it recognizes that we are in the U.S.A. and prices are for U.S. dollars? Thanks.