What if a country does not have any regulation about it?

Hello everyone.
This is my first post here and I know am in the right place to ask for advice.
Below I will explain my situation:

I have been doing bitcoin exchange for 1 year and 6 months. Till now I haven't had a problem with banks and taxes. I just meet with my clients, receive the bitcoins, give them the money and then use Bitstamp to transfer the money to my bank account. My monthly volume was 2000-3000 US Dollars.
Yesterday, a guy whom I met in a bar (friend of a friend) asked me if I was willing to buy him bitcoins. Of course, I said yes. He was willing to sell me 10-20k$ of bitcoins per week. Now I am thinking...

How can I verify that my profit is 5% of that sum of money and then I can pay taxes for my profit? The seller identity should remain safe of course. This is one of the main reasons people sell btc for cash.

So if I get 40k$ a month in my bank account, how can I verify that my profit is 2000$.

There are no laws for bitcoin in my country, and I don't think buying in cash is illegal.

Thank you.