You're best to sell on Amazon, so many people aren't using eBay cause there's a high risk of people getting ripped off. I don't use eBay any more, I've been there so many times and got ripped off.
What are the best ways to sell iPhone Online?
You have a few different options when it comes to selling your iPhone online, and most of them will offer you some form of seller protection.
Before making a decision you should consider which site you will most likely get the best price for your phone, this will probably be the site with the most users. You might also want to think about seller fees- how much you have to pay to list the item, and whether the buyer will collect, or you will be expected to mail the item.
eBay. Personally I have always used eBay to sell things. It is set up specifically for the purpose of selling items you don't use anymore, so has the necessary seller protection options, and is very intuitive for listing and checking your auctions.
Gumtree. Free to use and sell most items, but is used for many other different things (eg. Listing flatmates, jobs and so on) so does not have as many seller features.It is much like a local classified listing, so you're ad will be seen only by people in your area and they will collect the phone.
Craigslist. Very similar to Gumtree, feature wise. Is extremely popular in the States, but if you're not in the US chances are not many people use it.
There are third parties that will purchase your old iPhone handset, and your phone carrier may even provide this service - so it's worth investigating. Just be wary that anyone who helps you sell your phone will be looking for a cut, so compare what percentage they ask for.
When posting your ad, make sure to include an image and as much details as you possibly can (including any scratches, faults or flaws). The last thing you want is a negative rating or a buyer on your case because they misunderstood your ad.