I'm thinking of selling a necklace that my friend left behind at my house and never came to get back after 4 years
Why don't you just give it back to them yourself?
I just don't want to, that's why!
The problem is it's not yours to sell.  You can't put something up for sale that doesn't belong to you.  Why not just reach out to your friend and see if they're okay with you selling it, at least?
Are you close minded? It's still my necklace since I have it in my possession and it's still in my house.
It might be in your possession, but you aren't the rightful owner. You should at least attempt to contact your friend to find out if they still want it back first. I'm sure your friend would appreciate it!