I bought my iPhone from a third party and I cannot get in touch with the original owner what do I do now

iCloud: Activation Lock - http://support.apple.com/kb/HT201365 - This is a security feature to deter re-selling of stolen devices.

Turn off Find My iPhone Activation Lock (formerly: Activation Lock: Removing a device from a previous owner’s account) - https://support.apple.com/HT201441 - If an activation lock was put on a device, only the owner of the account that applied the lock can remove it.

Get them to remove it or return the device for a refund.

Yup,

If the iPhone is Activation Locked to another user/Apple ID and you have no way to contact them, return the iPhone to the third party you purchased this iPhone from and get your money back.

There is absolutely NO way to defeat Activation Lock!

Even Apple will NOT deactivate Activation Lock if you are not the iPhone’s original owner.

Return the iPhone and get your money back!

Good Luck to You!

If you cannot get in touch with the third party, as you say, there is nothing you can do.

I'm afraid you have lost your money.