You can purchase your ticket on klm.com (or klm.nl) with no problem. It is totally legal and legit. Prices vary according to the country in which the reservation is ticketed/purchased. I usually hold my reservation until I get to Schiphol, and then pay cash for it. This way, I am physically in the country, so there's absolutely no doubt.
The country in which the credit card is issued is irrelevant. If your card is physically present with you in Europe, you should be able to use it to get the regular local price.