The major drawbacks are ..
Your shopping from a store which is not genuine...
Delay in delivery of products...
Possibilities that the Product when delivered.. wont be the same what we bought online!!
Threat of hacking..
bu to avoid all these situation ...
you should first go through the websites privacy policy ...
check their reviews online ..
chat to the person who is handling the website... normally every e commerce store has a chat option...
Keep the product track id and bills safe !!

Go for branded store.. or big stores like amazon, ebay, etc