user66003: I understand sites like koinex/coinbase/etc.. credit bit coins when we try to buy bitcoins from them. 
How do they (koinex/coinbase/etc..) get bitcoins to give it to users when they buy? are they doing mining and getting coins? or do they buy bitcoins from somebody who is mining ?

user63569: No, simply they sell and buy bitcoins. I sell them my bitcoins and you buy from them these bitcoins.

66003: <p>I understand sites like koinex/coinbase/etc.. credit bit coins when we try to buy bitcoins from them. </p>

<p>How do they (koinex/coinbase/etc..) get bitcoins to give it to users when they buy? are they doing mining and getting coins? or do they buy bitcoins from somebody who is mining ?</p>
 63569: <p>No, simply they sell and buy bitcoins. I sell them my bitcoins and you buy from them these bitcoins.</p>
