Confirming used for sale device isn’t stolen

Not really. There is no list or database of stolen devices to check.

The best you can do is verify it has been restored to factory settings and can be setup by you. It should be at the Hello Screen.

If it's at any other screen, it has not been restored, and will likely be unusable.

The seller should follow the steps here:

Link-> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Verizon is relying on the USA cellular trade association (the CTIA) which manages the universal USA blacklist -> https://stolenphonechecker.org/spc/

But note that database is specific only to the USA, and only reports devices that in turn have been reported stolen to their own carrier to get the device put into the database. So it is not a guarantee that any given device is not stolen just because it is not listed in the CTIA database.

The really is no way to be 100% sure that a used device is not stolen. And note if the owner reports it stolen after you’ve bought it, it may cease to work if it gets blacklisted.

As mentioned not really (that Verizon system you used is not 100% accurate fyi). You can protect yourself by only purchasing from sites that have buyer protection and using payment methods that offer protection in case something you buy isn’t as described or was sold to you via fraud (like knowingly selling a device that can’t be used by the buyer)