Blockchain.info provides a nice to use feature "WatchOnly Address"
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Watch Only - Watch Only addresses allow you to view transactions at an address without storing the private key in your wallet.
With use of watch only address you can add address to your wallet to keep track of its balance. This is useful when you have offline wallet and you would like to see its balance in your blockchain.info account. This is possible because blockchain.info will also add balance of this address to your wallet balance and will show it as total balance in your account. For example, you have balance 2 BTC in your wallet and you add a watchonly address having 3 BTC balance , blockchain.info will show you 5 BTC as your total balance. Good.
Since it is just an address added to your wallet and not actual private key you will not be able to spend bitcoins from this address. If you will try to spend, it will ask you to provide private key.
But this can be, and already, used to scam newbies
Scammer creates a brand new blockchain.info account and adds an address as watchonly. So now it will show you balance of that address as balance in the account.
Scamer confirms trade with some one for BTC. Scammer says to seller that he is a noob and dont know how to operate blockchain.info account. He gives account access to seller. Scammer asks seller that he will give access to his blockchain.info wallet seller and seller sends him agreed upon goods ( most probably LTC or any other crypto currency ). Seller checks account and sends him goods. Seller tries to withdraw BTC and he can not.
Normally new users gets stuck with this. One of my customer got this issue.
Blockchain.info should show the balance of watchonly address as pending balance or something else. It should not be added to balance of that wallet. May be a separate account page to manage watchonly address and check its balance with total wallet balance.
In any case you should never trust wallet coming from other party as he may always have access to private keys. Always make sure bitcoins are transferred to address under your control only before completing the deal.
Cheers
