Best secure app/site to sell cellphone + what are signs of a scam?

I want to sell an iphone but I don't want to be scammed either since it's my first time. What are some websites/apps you ve had good experiences with and what are some warning signs I should look out for a scam?
Also, if someone says their only way of paying me is with a personal check, is that legit? Should I be wary?

Access iPhone 8 without pass code or connecting to itunes?

My grandpa found an iPhone 8 he thought he lost, and found it dead. He can't remember his password and if I try to plug it into my computer it shows an error message saying I need the pass code to use an accessory to an outside source like a PC. It won't allow me to access the camera so any of those tricks won't work. I can't hard restart by holding the power button either, because when it turns back on its disabled for an hour.

How much would an iPhone 8 Plus with messed up screen colors sell for?

Hi so my iPhone 8 plus is perfectly fine, actually only 6 months old except I cracked it - I was cheap and decided to go to some other store where I paid ~$140 to get it fixed, and it's fine except the screen resolution is duller and the colors are a bit inverted? I don't know, i hate it, and instead of paying $200 to go to apple to fix it, how much do you think I should rightly price it at to sell?

Will a glass or plastic film phone BACK protector stay on the phone securely whilst using a Spigen ring?

Does the protector adhere well enough that it won't come off of the phone when using the ring for what it's intended for? Will constant tugging of the ring cause bubbles under the glass protector or damage the plastic film?

Examples would be glass back protector on the all glass iPhone back, or in my situation a plastic wet film installed on the "matte" glass back of the Pixel 3… With a Spigen ring installed onto the protector.

What wrong with my IPhone?

So I broke my previous Iphone and mt friend was generous enough to give me her old one (Iphone 7plus) but I had to figure out her password because she forgot and then I could have it. So I didn't figure it out but I gave it to my OTHER friend to break into it cos she's good at that stuff and she only got so far that the Iphone restarted and i was pretty happy about that. The problem is, is that she only got so far and she said after she tried to get into it farther so that its restarted all the way but she could only get it to the screen that asks if you want to set up the phone manually. After setting it up (logging into Wi-Fi and choosing language ect.) It stops at the screen where it says
"Unable to Activate
An update is required to activate you Iphone
Learn more at hduwjfnsloxd"
And i tried to call and ask an apple employee but they just told me to ask my friend to go to iforgot.apple.com
but i no longer have connection with my friend that gave me the phone so i can't do this.
Is there any ways to fix this problem? I really don't want to buy another phone i got bills to pay