Query related to gst on purchase from overseas

I am purchaing goods from overseas which comes to me by post, and so there is no tax payment at any stage.

Now I am selling this goods on ecommerce website.

Here I know that how I should show my sales on GST retrun, but I am confused about my purchaes as it is imported but there is no payment of duty or GST at any stage.

Also late me know whether is it legal or not?

Note: I order only 1 pc electronics goods at a time and order value never exeeds to Rs.5000, I order other set once previous order gets dispatched. and sometimes postal custom checks my parcel but later they let it clear by marking duty free.

Expert Advice needed.