1. For an want-to-be online seller who intends to sell using portals like Flipkart, Amazon,is an IEC required?
IEC or Import Export Code is required when you are engaged in Import/Export trade. For trading within India, IEC is not required.
2. For an want-to-be online seller who intends to sell using portals like Flipkart, Amazon, what are all taxes including GST, an individual seller has to pay under the following conditions:
a. Products which never come in India, instead, are sourced from and sold outside India
b. Products which are sourced from outside India and are sold in India
c. Products which are sourced from India and are sold overseas
This is challenging part, you better ask a separate question for it.
2. Is it possible to achieve all the 3 types of sales mentioned above as well as inter-state sales within India with a single GSTIN?
Yes it is possible to do all above transactions with a single GSTIN. But you should have your godown/office in a single state, where you are registered.
GST registration is for each state, so if you open office in more than 1 state, you need to have registration in each state.
3. Does a seller still need a TAN even if s/he is a individual seller?
TAN comes under Income Tax law and not GST. TAN is compulsory to deduct TDS and payment to government.
If you are required to deduct TDS, then you need to have TAN. For example, if you are audited under income tax, and paying monthly rent of 40,000 then you are required to deduct TDS from rent amount, in this case you need to have TAN.
4. "Returns" as referred to in many places does not clarify what the individual is expected to do or what it actually refers to. Is there a process flow to explain how this works and at every step, what is the requirement, liabilities and benefits?
I believe you are referring to GST returns and no sales returns.
You need to file 2 returns every month (as on this day, GST council is considering launching a single return).
GSTR-3B, which is to report your sales, purchase etc. and payment of tax to government.
GSTR-1 to provide details of all your bills for the month. Further GSTR-1 can be filed quarterly if turnover is below 1.5 crore in a year.