What to do if I purchased a used iPhone and it's locked?

If you buy an Activation Locked iPhone on eBay, you cannot bypass activation lock without assistance of the previous owner. If you cannot get their assistance you option would be to try to get a refund from the seller or from eBay Buyer Protection. Also see: Activation Lock for iPhone, iPad, and iPod touch - Apple Support

What is the exact error message that you are seeing on your iPhone that you purchased on eBay?

Axel F.

Do not ever buy a phone from eBay. If it has an activation lock, you will never be able to use it, so don’t waste your time and money.

Return the phone for a refund. You won't be able to unlock it.

File a claim with eBay's buyer protection program. You've been ripped off.

Contact eBay. See: https://pages.ebay.com/ebay-money-back-guarantee/