I have found Swappa to be the best place to sell, but it will be flooded with people selling their old phones right now. Their fees comes to be around 10% of sale price and you are responsible for shipping with insurance. I have tried Mercari before, but that one is abour 14% in fees. ebay is likely even higher. There are other places you can trade in (Gazelle, etc.), but don't give the best price.