Recently I had to buy something on Steam while in another Country. Turns out it was impossible, because the country information of my Payment method differs from the Country selected based on my IP Address. So I contacted the Steam support, and they advised me to pass a special Argument to the URL when visiting the Steam Store through a Webbrowser:
http://store.steampowered.com/?cc=<countrycode>
Replace <countrycode>
with a 2 character country code, for example:
http://store.steampowered.com/?cc=DE