I bought a used iPad at the flea market. I reset it to factory and now I have activation lock on I don’t know original owner name

If you have bought 2nd hand iPad, you must contact the seller (hopefully it is the previous owner), otherwise you have been scammed.

Each Apple device is linked/associated with an Apple ID of the person who did the setup.

Activation Lock prevents anyone who is not the owner from using this iPad.

To unlock the Pad, you need the Apple ID and password of the previous owner, that were used during set up.

So you have to contact the previous owner.

The previous owner has to read -> this Apple article: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch -> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Then the previous owner has to remove the iPad from his account, see this Apple article:

Check your Apple ID device list to see where you're signed in

Check your Apple ID device list to find where you're signed in - Apple Support

Then you can sign in with your own Apple ID.

See the following Apple articles:

Activation Lock for iPhone, iPad, and iPod touch

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

How to remove Activation Lock -> How to remove Activation Lock – Apple Support (UK)

• If you need help removing Activation Lock and have proof of purchase documentation, you can start an Activation Lock support request.

Read this before you buy a newer iPad.

You can also contact Apple support for help, -> Get Support (apple.com)

You have to show proof of ownership of your product, provide all documentation showing your purchase of the device.

If you do not have additional proof of ownership, contact the reseller. They may be able to provide the original purchase document.

Once Apple receive your proof of ownership, Apple will further review your request and provide an update.

See also -> Keep your receipt after you buy an Apple device

Keep your receipt after you buy an Apple device – Apple Support (UK)

Ask your money back if possible, perhaps you have bought a stolen iPhone.

If you can't get in touch with the previous owner, you can't use the iPhone.

If the iPad disabled or Activation Locked, unless you can independently obtain assistance from the previous owner, the iPad is (and will remain) useless to you. If you are able to contact the original owner, they will require this information:

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

Here are the Apple support pages that outline the Activation Lock - and the only mechanisms by which it can be satisfied:

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

Turn off Activation Lock – Apple Support

As you will discover, unless you are the owner of the AppleID that was last used to activate the device, you’ll not be able to defeat or bypass the Activation Lock. With documentary evidence that you are the original owner (such as the original sales receipt showing the serial number), Apple may be prepared to assist - otherwise they and nobody here will be able to help you.

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

Here is what Apple suggests when purchasing a previously used device:

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.

You can’t; you have been scammed and almost certainly have stolen property . It’s useless to you