Buying products over the Internet using a credit card is actually less risky than using it in a restaurant, as FishNose points out. Most Poser stuff is sold via an intermediary, such as Renderosity. The merchant will never have access to your card number, only Renderosity (or whatever Poser site you are buying from) will. All Poser sites I have been buying from use a secure connection, meaning that your credit card info is encrypted during transmission for added security. Look for the little padlock icon in the status bar of Internet Explorer, it means encrypted connection, it should be there when typing in the card number. Or look in the address window, the address should start with https:// (not http://), it also indicates encrypted connection.

The only safety issue I have encountered with buying Poser stuff is that it fuels your Poser addiction and makes you buy even more stuff! ;-)