About on-line shopping

Nowadays , on-line shopping is popular in many developed countries.I never did it , so I don't know its advantages or disadvantages . I heard that shopping on line is more cheap , but not safe indeed.Because you never know the seller or see the product ,you may buy a fake .Did you do on-line shopping ever?Please give ur words !  How can we do safe shopping on line

For me, I trust some wel-known internet shopping mall in Korea. I bought some books and an eletric dictionary. Yes, it's cheaper than off-line shopping mall.

However, there are some other shopping mall which I am hard to trust in terms of safty.