Credit card companies protect you from fraud. you only need to review your credit card bill and if something is fishy, report it. they can take the money back from the seller, it's called a charge-back. it is the responsibility of the retailer to check that the person paying with the credit card is in fact the legititimate owner of it (signature match), to avoid getting chargebacks.
That being said, I don't fear using my credit card online. As long as it is a reputable store, there is no greater chance of having the number stolen than at a restaurant (do you really know what they do with it when they take your card?) or gasoline pump.