I bought an IPAD from the flea market and the seller does not know the previous owner. How can I erase all the details?

Sorry, the iPad is useless to you...

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

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, or unless you can secure the cooperation of the previous owner, you’ll not be able to defeat or bypass the Activation Lock. Without documentary evidence that you are the original owner (such as sales receipt showing the serial number), Apple won’t assist - and nobody here will be able to help you.

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

You have encountered Activation Lock a theft prevention feature. The Apple ID and Password of the original owner are required to open the lock. Without this information, you will never be able to use the device. If you cannot return it for money back, you can recycle it at any Apple Store.