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?

Zero.

Either have the screen replaced with the correct one or recycle the whole thing.

It's trash, I don't care how much it cost new. Apple got you. The battery has a short and finite life that repairing the screen will not change. You got what you paid too much for and ruined it.

Apple will charge $200 to fix it. But potential buyers will want a *LOT* more than a $200 deduction from whatever the going rate is for the exact same used phone in perfect condition.

That's because with the screen as it is, they can only take your word that's the cause. Also people selling accident damaged phones have a nasty habit of also telling their insurers that those phones have been lost or stolen, so purchasers soon find the phone they bought ends up iCloud locked and network blocked.

So at present, unless you find an idiot with more sense than money, your phone is worth little more than scrap value.

Unless you sell it to a phone selling company (that can fix it), you'll probably not be able to sell a broken phone.