Some require customers to do ID verification, while some don't.
Could you tell which ones do not require ID to verification?
The above mentioned site like coinbase btc-e . blockchain info localbitcoin all these sites dont require ID verification. ID verification is only requires when you buy bitcoin using these site espacially coinbase through bank or card.
For Coinbase, you will need to at least verify your phone number and bank account, or link your credit card.
https://support.coinbase.com/customer/portal/articles/1635397-what-are-user-verifications-used-for-
For btc-e, you won't need to do verification unless you need to make bank wire transfers.
Blockchain.info is just a wallet and a blockchain explorer, and there is no way you can buy or sell your bitcoin there.