Unfortunately, you have few options available to you. Apple will not unlock the device for you, and you cannot wipe an iCloud locked device. Unless you can get the original owner to login and deactivate Find My iPhone, there is nothing you can do with this secondhand iPhone.
Return your device for a full refund, and in the future, be wary of secondhand iPhone purchases. This whole matter is a shame really, because at least once a day, someone makes a post similar to yours, not knowing that the previous owner has to clear their account from the device to make it usable to another party.
If you are looking for an iPhone that suits your budgetary concerns, I recommend the iPhone SE, which retails for an incredible value of $399 for the 16GB model and $499 for the 64GB model, making the iPhone SE not only the most powerful 4 inch smartphone on the market, but the most accessible iPhone yet.