Income tax filing without purchase bills

Hi,

I am planning to do a business which involves antiques/numismatics (old coins and currencies).  The problem in this business is no one will give any bill when I purchase the materials and I have to pay them in cash.  About 50% of the time I have to deal with individuals for whom it is just a one time small selling and so it is not fair to expect them to give any bill too, but all my selling will be done through bank transactions and everything will be accounted.  I want to file for income tax properly and regularly, so I have the following questions.

1.  If my turnover is below 9 lakhs per financial year through my savings account, how can I file income income tax without any purchase bills.  How can I show the amount I invested through purchase and the amount I gained?

2.  In case of less than 9 lakhs turnover per financial year through my savings account, what will happen if I do not file for income tax?

3.  I do not want to go for GST number as of now as my turnover will be less than 18 lakhs for sure.  In this case, how can I go for income tax filing without any purchase bills with minimum tax or without tax at all?

4.  If income tax filing is possible without bills, how is it done and for how much amount per financial year?

5.  I may also need to send the materials to buyers in other states, will not having GST have any bearing on this?

Thanks in advance.