Welcome to the community!
a) If we are buying Bitcoin from any of this exchanges like CoinSecure / Btcxindia.com and then when I want to sell it - is it guaranteed that I would be able to sell it ? Is there a possibility that after buying, I am not able to sell and get stuck with them ?
You won't be stuck with them, unless something really bad happens, like banning BTC in India or leak in bitcoin security.
b)Though it's a bitcoin forum - any of you have experience with Ethereum ? How do you see the value of Ethereum moving ?
I haven't dealt a lot in ETH, pump and dump still in force with this one!
c) Any of you have specific experience with BtcxIndia.com and EthexIndia.com that you can share ?
Unprofessional & high fees, I'll suggest to check for alternative.
d) In India are there any specific tax implications that I should be aware of while trading Bitcoins ?
Right now the law area is a bit murky in India. You shall pay taxes on profit you made from trading cryptos. Check with a tax consultant if your income shall fall under PGBP or Capital gains or IOS.
Bitcoin is sold in India at a premium fluctuating from 10-30 %
Trading fees are still high HODL, and we shall be rich soon
Good luck!