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This a thing, I've seen rewards in the range of 1-20 BTC posted in other forums. Once you have control of the wallet, that user could deduct whatever they want. It's a risk for both parties.

that is assuming there is something to break! if i am right and this is indeed a wallet that is being sold, then it means it is a scam and it contains no coins whatsoever. it certainly is not going to be the first time something like this happens.