how do I know if an iPhone XR is new or refurbished

Another thing of which you need to be aware as we see not infrequent posts about used devices being locked:

Find My iPhone Activation Lock - http://support.apple.com/kb/HT201365 - This is a security feature, presumably in part to deter re-selling of stolen devices.

Turn off Find My iPhone Activation Lock (formerly: Activation Lock: Removing a device from a previous owner’s account) - https://support.apple.com/HT201441 - An activation lock can only be removed using the log-in information for the account that applied the lock.

Get the owner of the account to remove the lock or return the device for a refund. It is otherwise useless to you.

If the owner has not provided contact information as a message showing in the login screen, there is no way to contact the owner unless the person who gave you the device knows them.

Apple may remove a lock if you can provide sufficient evidence of valid ownership such as providing the original sales receipt, preferably one with your name on it. You must take the device to an Apple Store along with required documentation. There is apparently no set policy (I asked at an Apple Store) and a store may simply refuse, particularly if they are not satisfied you have proven ownership.

Just beware it could still bite you. A common eBay scam is to take a carrier offered upgrade, but not return the old phone as dictated by the terms of the upgrade. Scammer immediately then sells new, unopened device for cash, then reports it stolen, which gets it blacklisted. Or, the carrier, upon not getting their money or device that is due, cancels the account and blacklists all devices associated with that account holder.

Anyone who legitimately changed their mind with a brand new device could simply return it to the carrier (or if from Apple, right back to Apple within 14 days) for a full refund.

It's unlikely that it's refurbished. However, the fact that it is being sold on eBay means it is not "new". It's used. Even if the box was never opened.

Just like a car. If you buy an untitled new car from the dealer. If you buy a new car from a dealer, then immediately sell it to someone else, even if you never even started the car or put miles on it, it is used.

The warranty started the day the device was originally sold by Apple or an authorized retailer.

If it is being sold at a significant discount compared to purchasing directly from Apple, something is wrong. Supply is too limited and sales on iPhones are rare. Don't buy it.