It's safe and convenience is its middle name. Ensure that you know how much you're paying, including delivery charges, before you enter your credit card information. That way you don't get any nasty surprises. Shop at established sites by reputed brands. With the entry barriers being so low, almost anyone can open shop on the Web.
Looks can be deceptive in cyberspace too.
Reputed promoters and partners are a safe bet. Ensure that you enter your credit card information only on a secured page - browsers normally display a message informing you that you are entering a secure page. If you're using Netscape, the icon in the corner showing a full key or a closed lock indicates just that. Note down contact information of the vendor - physical address, email address, phone and fax numbers. This comes handy in case you need to get in touch with someone from the online store.