Activation lock

Probably you are referring to Activation Lock.

If you are the owner or the owner has shared all information with you then to open Activation Lock:- If it is Activation locked, read here --> Turn off Activation Lock - Apple Support Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Learn how to remove a device from a previous owner's account.

If you are not the owner and do not have the Apple ID and Password then there is no way that you can get into this iPhone unless the original owner shares the Apple ID and password with you. If you are lucky you can get into this iPhone if they (owner) accidentally remove the device from their Apple ID.

If it was not disclosed in the sale that it was activation locked or iCloud locked, return it for a refund. If the seller refuses, file a claim under eBay's buyer protection program.

There is no way to bypass the activation lock. It is useless.

If it was disclosed in the item description on eBay, you bought a paper-weight. There is no way for you to get the activation lock removed.