Is it possible to add a used Sprint phone to a Sprint family plan?

I cannot speak to Sprint. However, my cell provider does allow used phones to be added. My son broke his phone so my BIL gave me an old one from the same provider. We had no problem with it. You could call Sprint and ask them if they will add a used phone to the plan. You do not have to mention eBay. They should be able to tell you and they may have specific things you need to make sure are included with the phone (or not included.) Some phones require an increase in service like a data plan.

of course you can add it. as long as it is a phone they can provision on their network.

I thought about giving my mom my old phone and adding her to my family plan and Sprint said I could. That was a few months ago, not sure if something has changed. I was recently in a sprint store and they had some cheap phones I thought.