Technically you can sell it (specifically Verizon) because in most cases the carrier lock is automatically removed after 60 days (even if you are on device payment plan). If you don’t go ahead and pay the balance off though you will still owe it. The buyer wouldn’t have any issues using the phone (again, this is assuming the carrier lock is gone which you can check yourself in settings). I have done this often in the past when I would sell the iPhone myself knowing I could get more than trade in value or what was owed on it and then would pay the balance off (usually after the buyer has activated the phone on their own account and all is proven to be well). Different circumstance though which may make your situation different.
There is also the caveat that the line still exists on your account and either doesn’t have an active phone on it or you are trying to sell it while it is still showing as the device on that line. That might very well be where your issue would come in. If you still have it active on your Verizon line the buyer would only be able to use the extra eSIM to add a second line (not sure if that would need to be in your name since the primary active line is in your name). If you contact Verizon to cancel that line the balance of the phone would be due immediately. That would mean either you need to handle that first and truly free up the device (which would make things easiest) or you would need to have the buyer pay you first and trust you will pay the balance so that it is not at risk of being blacklisted due to non-payment).
So the simplest thing for you to do would be to go ahead and pay the balance of the phone off and then sell it. Or you could utilize the option of transferring the line over to someone interested in it. They would then become the owner of that line and the device payment plan agreement.
https://www.verizon.com/support/transfer-your-service-mva-video/#:~:text=You%27re%20responsible%20for%20charges,to%20the%20wireless%20number%20too.
Or…you can add a different phone to the line as a place holder, sell the iPhone, and then immediately pay off the balance (but be prepared that you may not be able to sell it for the amount you owe…depending on condition, age, and storage. Unless you are selling it with an active AppleCare plan you aren’t likely to get what you owe. So make sure you can cover the difference if you need to).
