Buying online. How do i know which websites to trust?

So i've been looking for a camera lens for my nikon DSLR and i came across this website.http://cameraslamps.info/ (wwwDOTcameraslampsDOTinfo if the link doesn't work) And their prices are really cheap. How do i know which online stores to trust? If i pay through paypal will they help me reconver my money if it ends up being a scam?

Added (1). Sorry i should have mentioned that i know this particular site is a scam, they ask for Westernunion Payment. But i mean other websites. I generally go buy if they are with paypal or not.

You never know fully I suggest looking for reviews on the company your looking at from other people
If they are a reptuable company they will always refund but if their not then probally won't

Always just do review checks

It's a 3 month old Chinese site http://whois.domaintools.com/cameraslamps.info

I personally won't buy from sites that are less than a year old or are located in countries known for fraud and counterfeit items