There is nothing to be done.
If you cannot elicit the help of the person that sold you this iPad, then there is nothing more that can be done.
You purchased an expensive, useless device.
You got scammed.
The proper way to setup an iPad for sale greets you with an initial, animated, multi-language “Welcome to iPad” screen when you first power it on.
Should have checked for that by powering up this iPad before walking away from the person that sold you this iPad.
If you are able, attempt to get your money back!
Failing that, this iPad is a totally useless device to you.
An expensive lesson learned.
Please, recycle the device responsibly.
If you need/want an iPad with an up to date iPadOS with access to current, updated apps, then you really need to seriously consider a new iPad model OR a much, MUCH “newer” and less expensive, used iPad models, from better known, trusted Apple product/device retailers/resellers, capable of running the latest iPadOS versions.
There are plenty of newer, older iPad models that can run the latest, current versions of iPadOS.
So Sorry.