Why is there difference in buying and selling prices of bitcoins in unocoin ?

Zebpay and Unocoin both have a huge difference between their selling and buying price. This is called spread and helps them make big profits. And yes, you are correct that this is in addition to the fees. Similar behaviour can be noticed with Paypal which charges some fees and also has a lower exchange rate than the actual exchange rate.

If you want to avoid this loot go for Coinsecure. They have low fees and low trading margins.

However, do note that the behaviour of both Zebpay and Unocoin is not unique and the difference can be found in other exchanges too like in the US.