Were it me, I would wonder if the prior owner had something on the device that you don't really want. Hopefully you haven't put much on it yet.
I would suggest that you save your personal stuff, contacts, files, info, etc. (just not apps as you can't save those) to a computer or cloud account. Then wipe the phone by doing a "Hard Reset" (instructions below). Just make sure that it is only your info that is the main google account attached to the phone before doing this. If not, then you risk losing complete access to it.
Once you have reset it, anything that someone could have placed on the phone would be removed. NOTE: This will only work with a full "Hard Reset". Trying the option that saves anything on the phone won't work as it would not remove the problem.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the phone...
1. Make sure that the phone is actually powered off.
2. Remove your SD card, if there is one in the device.
3. Press the "Power" and the "Volume Up" buttons, together, until you see the start-up logo on the screen.
4. Once you can see the logo, release the buttons and allow the device to enter the "System Recovery Mode".
5. Next use "Volume Down" button to get to the "wipe data/factory reset" option.
6. When in recovery mode the "Volume Up" and "Volume Down" buttons are how to can navigate and highlight what option you need.
7. Once you have selected the "wipe data/factory reset", press the "Power" button to select that option.
8. On the following screen, repeatedly press the "Volume Down" button until the "Yes - delete all user data" option is what is selected.
9. Then press the "Power" button to hard reset the phone.
10. If you took your SD card out in step 2, when the device is up and running again, reinsert the SD card.