If that happened to me, I would triple-click the home button. It sounds like you have some Accessibility feature that is turned on and you need to get it turned off.
Once you solve the accessibility issue, force restart the iPad. Home the home and on/off buttons together for about 10 seconds until the Apple logo appears.
Further issues are likely, however, as the previous owner clearly did not do such a good job of thoroughly cleaning the iPad.
What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support
You may even find the iPad has an Activation Lock, which requires the previous owner to unlock, or the iPad remains useless to you.