Smart phone can be easily stolen and there is been increase in smart phone theft every where. Big smart phone companies are lunching new features to protect their users and as Apple recently added security feature to their new iOS7. This security feature save the owners Apple ID secretly to the OS as the owner Turn On the Find My iPhone. If any theft or any other user try to use iCloud it will ask to activate phone through previous owner Apple ID.
If you buying used iPhone then first you should check and Turn On the Find My iPhone. If you stuck at iCloud activation then unfortunately there is no free tool available to bypass except for the iPhone 4. There are few ways to bypass iCloud of iPhone 4 for free that you could find here. There are many free tool available for other iPhone version that ask for survey are fake. Few users give service for more then $100 that take more than week but still they are not trustable.
You should check iCloud activation before buying used iPhone. There is free tool Doulci that is in beta and many users claim that it work for them so still we need to wait for any tool. For more information related to this problem you can find on Apple Support.