That just means that your device is in a recovery loop. Download iTunes, and plug the device into your computer. iTunes should pop-up if it's not already opened after installation, and will prompt you to restore the device. After restoring the device, you should have access to the setup screen.
The one thing that I'm concerned is that the device is iCloud locked, and the seller scammed you by saying the device is fully working when in fact the device is unusable. You can't tell whether it's iCloud locked or not until you've done a restore and attempt to set up the device for the first time.