The Spanish phrase, "comprar en los Estados Unidos" means "buy in the United States" in English. The verb "comprar" can also be translated as "purchased," which is generally used in formal speech, according to Word Reference.
Country names are generally translated in Spanish, which includes "los Estados Unidos," meaning "the United States" in English. In regards to the phrase above, the Spanish preposition "en" can mean "in" or "on" in English; Merriam-Webster provides all of the possible translations and the manner in which each is appropriately used. In this case, "en" is translated to "in" as it follows the rule of location. Thus, the phrase "comprar en los Estados Unidos" can be read as "buy in the United States."