purchased used locked ipad how do i unlock it?

I suggest contacting/reaching out to Apple Product support and services.

Tap on the first link below and scroll down some to get to the Apple Product Support section and phone numbers for your location/region.

https://www.apple.com/contact/

OR

https://support.apple.com/en-us/HT201232

Tell them your story and see what Apple says for you to do.

Best of Luck to You!

Without the Apple ID and Password of the original user who set the Activation Lock, you will never be able to use the device. The only acceptable receipt is the original receipt clearly showing the serial number. Auction receipts are not original receipts. Here are links that will corroborate what MichelPM has told you.

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Turn off Activation Lock - Apple Support

The All Too Common SAD Reality of Buying … - Apple Community

You don't.

Without the access to the original users of these iPads, these iPad are worthless, useless devices.

You will never get these working.

No one here can help you. Apple will NOT help you.

You purchased a bunch of bricks/serving trays.

Never, EVER purchase unknown, used Apple mobile devices from auctions, thrift stores, flea markets, lawn/tag sales, online postings UNLESS you can turn on each device and it powers up with the initial Welcome to iPad with animated languages moving on the screen.

Just recycle these devices responsibly.

You'll never get these iPads to ever work for you.

Sorry.

Thank You.

Contact Apple support just to clarify this for yourself.

I do not think you are going to get these iPads unlocked, but actual Apple employees may tell you otherwise.

Best of Luck to You!

Great to learn!