hi, I intend to buy a second hand iPad, is it possible to know its original purchase date by the serial number?

No. Device Serial Numbers are in no way related to the date a device was purchased.

Be aware that purchasing a pre-owned iPad is fraught with risk - as many are Activation Locked. An Activation Locked device can only be recovered by the previous owner whose AppleID previously and successfully set-up the iPad.

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

Here is what Apple suggests when purchasing a 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.

If you decide to go down the path of purchasing used, secondhand Apple devices, here is how to avoid purchasing an Apple iOS/iPadOS device with user activation lock, in the future.

If you purchase any iPad, or any Apple iOS device, for that matter, used/second hand, online, or even in-person, make sure this iPad does NOT have user activation lock and has been properly reset for sale.

A properly reset for sale iPad should initially power up to an animated, multi-language “Welcome” screen.

DO NOT take the iPad seller's word that this iPad is user activation lock free!

We have seen plenty of used iOS device purchasers get scammed.

User Activation Lock can never be bypasss, circumvented or worked around IF there is no correct previous/original user information to be able to do so.

Apple doesn't get involved with ANY user activation locked, used/second-hand sold devices, either.

So, buyer beware!

If purchasing a used device in-person, power the iPad ON

If the iPad DOES NOT display the initial animated, multi-language “Welcome” screen, then pass on purchasing this iPad.

If the iPad powers up to any other screen, then forget purchasing this iPad.

You cannot trust that the seller actually has the correct needed info to properly reset the iPad back to factory settings right then and there before the actual purchase and exchange of money for the iPad, in question.

Pass on the device!

If you are purchasing this used iPad online ask the seller to provide multiple images, either online or be sent to you via email, showing this animated, multi-language “Welcome” screen of the interested in purchasing iPad model.

Even better, ask for a short video be sent to you showing this animated, multi-language “Welcome” screen, in action and in its entirety, posted to the online ad or emailed to you.

If the online iPad seller refuses any of these requests, pass on the iPad. DO NOT buy!

Good Luck to You!

you can try https://checkcoverage.apple.com/