It can save you quite a bit if you make international purchases regularly.
How?
Both
Amazon and
ebay (and presumably other retailers) skim off the top when converting from other currencies to the U.S. dollar (and presumably in the opposite direction as well), so using their systems to charge your credit card in U.S. dollars gets you a worse conversion rate than your credit card will, if you just charge in the native currency.
Now if your credit card company charges a currency conversion fee or international transaction fee, all bets are off (most credit cards DO charge for these things), but if you can find a card that doesn't charge for international tranactions or currency conversions you can cumulatively save quite a bit over time. I have an "
Amazon Visa" (not the same thing as the
Amazon Store Card) that I only use for international transactions, as it doesn't charge a currency conversion fee.
Example from this evening: I made a purchase from a Canadian seller on
ebay. The total was just over C$70.
ebay defaulted to charging $55.71 in U.S. dollars. When I changed the options to charge in Canadian dollars to my
Amazon visa card, the amount that hit my card was US$53.96, or a savings of just under US$2 on this one transaction. I know that doesn't seem like much, but it can add up over time.
By default, both
Amazon and
ebay will try to charge you in U.S. dollars so they get to keep that difference, so you have to take your time and doublecheck as you go through checkout.
I do enough international purchasing, not only through
ebay and
Amazon international sites, but also vendors like FNAC, JPC, Yvert, etc., that being able to charge in native currency has saved a ton of money over the years.