Buying ipad from a private seller

Note that even if you see the hello screen, that does NOT mean that it has been signed out of iCloud/find my. It may very well still be activation locked. There is no way to bypass the activation lock. Only the owner of the account used to lock it can remove the lock.

Here is what Apple suggests when purchasing used:

Check for Activation Lock before you buy a device from someone else

Before you buy an iPhone, iPad, or iPod touch, from someone other than Apple or an authorized Apple reseller, make sure that the device is erased and no longer linked to the previous owner’s account. Follow these steps to check if the device is ready to use: 

1. Turn on and unlock the device. 
2. If the passcode lock screen or the Home screen appears, the device hasn't been erased. Ask the seller to completely erase the device by going to Settings > General > Reset > Erase All Content and Settings. Don't take ownership of any used iPhone, iPad, or iPod touch until it's been erased.
3. Begin the device setup process.
4. If you're asked for the previous owner’s Apple ID and password, the device is still linked to their account. Hand the device back to the seller and ask them to enter their password. If the previous owner isn't present, contact them as soon as possible and ask them to use a browser to remove the device from their account. Don't take ownership of any used iPhone, iPad, or iPod touch until it has been removed from the previous owner’s account.

You will know that a device is ready for you to use when you're asked to "Set up your iPhone," “Set up your iPad," or “Set up your iPod” the first time you turn it on.

There is no database of stolen iPads. If you insist buying, then insist the seller follow these instructions:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

If he does, then you will see the "Hello" screen when you turn it on and you can set it up. If you don't, he did not prepare it correctly, so make sure you can retrn it for a refund.

Don't do it.