Your first enemy when purchasing online is your own computer: a simple malicious plugin you may have installed within your browser may compromise your privacy dramatically (ZombieBrowserPack is a plugin developed originally as a POC and which you can install on Internet Explorer, Firefox and Google Chrome but can be manipulated remotely to steal authentication credentials, and even bypass two-factor authentication mechanisms such as the ones implemented by Yahoo and Google. It can also hijack your Facebook account, or get your bank account credentials if you perform an online purchase. You can imagine such a plugin purposed to fulfill this or that feature (shim) and doing something nefarious behind in the background.) So you need to scan regularily your computer against the presence of spyware and other types of malware by using firewalls, antiviruses and keep your connections encrypted.
Also, you better need to type the URL of the website you want to purchase on yourself instead of searching for it and clicking on it in a forum page, in an email sent to you (even from a trusted person as email addresses can be spoofed or even hacked using a simple but effective technique such as password guessing) because an attacker can craft a malicious link that will lure you to a website that looks like the one you were used to purchase on but actually is not. Note that a secure website you may purchase on must use secure connections (https) looking something like this:
Avoid using your laptop to purchase something online whereas you're in a public place where nefarious people may be interested in the codes you type to valid your transaction. Same advice is valid in case your purchase with your phone. Better to be patient to do the transactions once you're at home.
Be always careful: attackers are used to send emails pretending to be from your bank or any service you may use and provide you crafted links to click on and pay for something. Read the emails carefully and use common sense to see if you're not going to be trapped. Do not forget to save records of all your transactions (print them for example) just in case ...