USPS stole my new iPad from eBay, reported about delivery and activated it. How and can find thief or location?

Unless it was you that set-up and activated the iPad, you’ll have no technical means by which to locate the iPad.

If you have yourself activated the iPad, here is Apple’s advice for lost or stolen devices:

If your iPhone, iPad or iPod touch is lost or stolen – Apple Support

Use the Find My app to locate a lost or stolen device – Apple Support

If the Find My iPad feature was enabled prior to it being “misplaced”, then if active, you would stand a good chance of locating it. 

However, if the feature was not activated prior to being misplaced - or the iPad has been inactive for more than 24 hours, the last known location will not be available - and the opportunity to use technical means to recover the iPad have been lost.

If someone other than yourself completed the initial setup - and activated the iPad - your only course of action is lodge a complaint with the Carrier and inform your local authorities.

How did you activate it when it was stolen?

You should contact ebay and USPS to report the theft.