My iPhone 6s got activation request denied, and I bought it second hand from ebay.

Sorry, you bought a brick. Apple will only unlock it with the original receipt, not a receipt from eBay as anyone could sell a stolen phone on eBay.

Activation lock is a security protocol that can't be bypassed. Only the person it is locked to can unlock it.

If you bought this from someone and it came locked, try to get your money back, you will never be able to use the phone.

If your phone is Activation Locked - How to remove Activation Lock -

How to remove Activation Lock - Apple Support

The All Too Common SAD Reality of Buying a used phone -

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

Apple will not unlock a device purchased from a third party. You will not be able to use this device. Your only option is to try and return it for a refund.

The Activation Lock is to prevent the device from being used if it is stolen. That is why Apple will not get involved in third party sales. You have no idea if it is stolen and maybe the original owner would like it back, since it is their phone. You should also know that the original owner is able to track the device still, so they will know you have it.

If you had your iPhone stolen would you want someone to be able to sell it on Ebay and have the buyer be able to use your phone?

I had trusted Apple to unlock the device

Why? Apple is under no obligation to unlock the phone.

The guy said that it was final decision but I don't take that for an answer.

Take it or leave it, you have the final answer. Apple will not reconsider their decision unless you have proof that you were the original owner of the phone.