Also another way to check is that your browser is set to secure HTTPS (not the HTTP). The only way your passwords, usernames and so on will be securely private. I hate it when websites / forums create their sites and I've seen a few that have no thought put into their "web" security... I just don't like it at all.
if you click on the "Secure" section with the padlock, you will find a little bit of information and also set certain websites not to play java, or whatever so applies. I hate websites that use trackers, I use a number of security software (Ghostery, Adblock Plus, Avast, so on) to prevent trackers, and to block malicious websites as I need my online banking secure and I can't afford to let my guard down with that.
Also if one looks for website reviews, you will definitely find a favourite or safe website to buy from. That's the only way I trust a seller in most cases specially when it comes to online shopping.
Last of all, if it appears too good to be true, then it is probably is. Trust your instincts.