What is the error message you see on your screen?
If the iPad is activation locked and you don't know the Apple ID or Password which was used to originally set up the iPad, you can't use it unless you can find the person who locked the iPad. Apple won't help you unless you are the original buyer (you're not) with the original purchase receipt clearly noting the iPad serial number, which you don't have.
If the iPad is disabled from not knowing the iPad Passcode, that will present another unique situation for you as the only way to get past that is to erase the iPad using your home computer with iTunes, BUT and this is a big BUT, you still need to know the Apple ID and Password that was used to set up the iPad, which you clearly don't know.
The bottom line is the store who sold you iPad did sell you a device which was properly prepared for a new user and you now own a useless device. They should make this right for you. We can't help with that.
See this link about Activation Lock --> Turn off Activation Lock - Apple Support
See this user tip I wrote about buying used iOS devices --> The All Too Common SAD Reality of Buying … - Apple Community