iPhone 5 locked out from previous user
Activation Lock has no documented bypass, so your primary course of action will be to file a claim with whomever received your payment to reverse the transaction if they cannot locate the person who has the lock on the device.


How can I bypass Activation Lock?


The Apple ID that has the lock can simply log into the web interface at iCloud and remove that device - so this is purely a contract law / human engineering issue where you will need to expend some effort to recoup your costs, get the seller (or agent that processed the funds) to reverse things.

Depending on how you paid and your local laws, a chargeback for false sale might be relatively straight forward for you. 

Ideally, sellers read the first article and buyers the parts of the second that cover purchasing a used iOS 7 device. You might find something in these articles to help build your claim if needed to the bank or seller if the ad you responded to was factually incorrect, misleading or otherwise false. 


http://support.apple.com/kb/HT5661
http://support.apple.com/kb/HT5818