If you had a debit card, they can't go over the balance of your checking account, obviously. If you have a gift card, they can't go over the limit there. However, if this is a regular credit card and you are in good standing, they will LET you exceed the limit to a certain extent. It is up to them because they can automatically give you a higher credit limit at any time if they want to. The limit is only fior their protection and not really yours.
So, if this is not a gift card and not a debit card, someone could go over the limit.
Many online vendors have a guarantee of some sort. "Verified by Visa". Or they are just plain hionest. Companies can loose their online reputation fast if they start jerking you around. So, you may be reasonably safe there. Where you could get into trouble is if, in the process of entering your card, someone intercepts your card information (type, number,security code).
Also, most credit card companies have fraud protection and limits. Usually, if you report fraud or misuse quickly, they will limit your liability to a certain figure like $50 or, even, $0. Your credit card information can be compromised anywhere, not just online. A waitress or store employee can make copies of your card information or double scan your card. Credit card fraud is everywhere.
I agree with the gift card idea that others have suggested but only for some unusual purchase or to a place you are not too sure of. Let me give you an example of what I did.
On the U.S. Post Office website, I opened an account to do a lot of shipping a couple of years ago. In the registration, they asked for UP TO three credit cards to keep on file. Some people will not do that, but I confirmed this is the post office and, aside from getting shot, I trust them. Since I had just scanned my computer for malware, I put in my card numbers. Why? It is a lot safer to put in your credit card numbers ONCE and not keep putting them in many, many times. You are more likely to have theft by malware on your computer than by misdeeds of a ligit business on the internet. So, if I know who I am dealing with and plan repeat businesses, I'll let them hold onto the numbers for me so I can say, "Just use the credit card with the last for digits ...1234" rather than type the whole thing again.
I've been doing online banking an eCommerce for awhile and I know how it feels to type your information into the form. Some suggestions:
1) For a single large transaction with a site you don't know, consider the pros and cons of using a "gift card" or your regulat credit card.
2) If you are going to do these purchases a lot, make sure your computer is squeeky-clean. Anti-Virus (good one), anti-malware, spyware. Make sure your rouer is running security.
3) Never buy online, unless you absolutely have to, from an unknown comuter (one that is not YOURS). If you have irresponsible people using the computer, either scan or don't use it.
4) Make sure communication from the start to finish, as well as any registration and login, is done with SSL. Your browser should have a little "lock" symbol and/or the website URL should start with "https://".
5) Make sure you trust the site. Is it Amazon? A site for a brick-and-morter store? You didn't click on a link in some strange email? The offer isn't "too good to be true"?
6) Consider storing your credit card information on your computer inder encrption (Roboform). That way, you can copy and paste the numbers and avoid a keylogger.
7) Open a Paypal account BUT only if you are quick to spot phony phishing email. Only go there if a purchase site sends you there or you key in the URL yourself. Never click on a link in an email to get to a site like that.
Go to the Mastercard or Visa or whatever website and see if you have any online protections you can sign up for such as "Verified by Visa" (but I have yet to see this in action). Sign up for alerts if you can.
9) Try not to worry too much. You should always check your statements, balances, etc. to look for anything suspicious and call your bank if you spot something shady quickly. Most banks will work with you to remove all fradulent charges.