When you purchase an iPhone with an iCloud lock, the only screen that you will be able to see is the iCloud Activation. It will ask you to type the Apple ID (or iCloud ID) and the password. The only way to bypass that screen is to type the Apple ID and password, but this information is only known by the Original owner of that iPhone.
The Activation Screen informs you that this iPhone is currently linked to an Apple ID. It will also ask you to sign in with the Apple ID that was used to set up this iPhone. The hint of the Activation Screen is only the first letter of the Apple ID and the domain of the e-mail service the original owner used ( Example: firstname.lastname@example.org ).
Of course, one way would be to type in all the possible combinations of an email address into the Activation Lock screen, but obviously that is not feasible and also Apple will block your IP after the third try.
How to find the iCloud ID/Apple ID of the original owner?
Apple has some tools and methods you can use to find the Apple ID of an iPhone, BUT they can only be helpful for the original owner (Find the Apple ID of my Device).
The only method that can help you to find the iCloud ID/Apple ID of the original owner, which most of the times is the original owner’s e-mail address is with the iCloud Contact Information Service. Not only it will provide you the Apple ID, but also all available contact information of him/her.
The iCloud Contact Information service helps to find the Apple ID/iCloud ID of an iPhone
iCloud Contact information service is supported by companies that have access to Apple’s iCloud Accounts Database, which allows them to locate all the information about the original owner of an iPhone, with the IMEI Number and the UDID number of an iPhone (UDID number is very easy to find).
The iCloud Contact Information Service will provide information about the original owner’s iCloud Account such as Account Full Name, Apple/iCloud ID email, Address, Phone & Secret Questions (Not Answers).
Sample Data of an iCloud Contact Information Service Report:
Backup email (not always)
Phone (not always)
Security questions (not always)
Ready to get the iCloud ID & contact the original iPhone owner?