How to sell my old iPhone 8+

If you sell it on eBay you will be guilty of fraud unless you properly prepare it for sale, trade-in, or giving away. Do the right thing. If you don’t do that the person who buys it will not be able to use it under any condition because of Activation Lock. You will be reported as a disreputable seller.

Sell, give away, or trade in your iPhone - Apple Support

Before you sell or give away the iPhone, be sure to prepare it properly for a new owner. Someone already posted the link, but here the links are, again.

Sell, give away, or trade in your iPhone - Apple Support

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

If you fail to do this, the next owner will be left with a phone they can never really use properly.

If the phone is carrier-locked (can only get service from a particular carrier / phone company) be sure to mention that in your ad, so that someone who does not use that carrier does not accidentally purchase a phone that they won't be able to use by mistake.

Bob Timmons wrote:

Take it to an Apple Store to see if they will give you a store credit. It might be worth $20-30 or so.

Apple's trade-in page says that they currently offer "up to $45 [USD]" in trade-in credit for the iPhone 8 and "up to $60 [USD]" in trade-in credit for the iPhone 8 Plus. These appear to be the oldest phones that Apple still accepts for trade-in, so the offers might disappear at any time.

Apple Trade In - Apple

If you're getting a new iPhone, you may find the following Support article helpful.

Use Quick Start to transfer data to a new iPhone or iPad - Apple Support

You could place the new iPhone near the old iPhone 8 Plus to do a transfer. Once the transfer was successful, you could then reset the old phone and prepare it for a new owner (as outlined previously).

Take it to an Apple Store to see if they will give you a store credit. It might be worth $20-30 or so.

Otherwise, sell the phone on eBay.com