This is the most commonly asked question. Everyone knows a story about somone who's been ripped off on the internet. But we all also know hundreds of people that have successfully completed transactions online. There are seveal ways to see if a website is legitimate. One is to go to the database of domain registrations and check the ownership deatils of the website: http://www.whois.net/whois.cgi2?d=matchingrx.com Don't trust a site that has an unlisted registration or a site that is registered in, for example, China or Togo.
A second way to insure that your credit card information is not being stolen is to check the payment gateway for authenticity. All payment gateways must have an address that starts with https:// The "s" in the address means that you are on a secure site.