From most of internet buyer experiences, there are 3 common problems which address the safety of online buying:

1. Ensuring the legitimacy of the seller
2. Ensuring the money is sent safely to the receiver(seller)
3. Ensuring the safety of the performed transaction

1. Ensuring the legitimacy of the seller
• Call the seller directly and have serious conversation in asking their legitimacy. Remember, it is your right to ask before committing any transaction
• 'Who Is' record to detect the ownership of any domain name is not a really safe method. Domain name owner can alter their 'Who is' record in just few minutes.
2. Ensuring the money is sent safely to the receiver(seller)
• Use escrow service in which you have the ability to block pending transaction if you suspect fraud during business deal
• Use reliable third party to handle your transaction such as Paypal, Worldpay, etc.
3. Ensuring the safety of the performed transaction
• Use reliable third party to handle your transaction such as Paypal, Worldpay, etc.
4. If the seller is using their own processing gateway, make sure the safety of the website (using SSL - the URL usually start with https prefix)

Can anyone explain on how people buy things online? Is it always not safe to buy things online? How to know that it is a safe shopping?

There are more scams on the internet than you could know.
Many people have been cheated through a fraudulent online shopping website. Why does it keep on happening?

Before you buy something from any website, please spend your time to read their TERMS AND CONDITIONS & PRIVACY POLICY first.
For example:
Muuah.com's TOS
But that doesn't guarantee you it is a safe trade. It might be a fake website just to get your sensitive information.

So, take some more time to Google on people reviews about that website. Put on some more effort to research on the background of the website owner too. Make sure they have a valid contact information.

And make sure you pay through Paypal. Have a safe trade friends! ^_^