It's amazing how much iPhones go for with dings and scratches on swappa. It doesn't kill value as much as people think. Most people who are buying jet black are aware of micro abrasions. Would it affect resale? Possible. But I don't see it killing it considering what I see sell and for how much.
iPhone 6S+ 64GB Verizon (they're Unlocked World Phones) are selling for $100+ more in "Good" or "Mint" condition than in "Fair" condition.
That's a 20% or greater price drop based on the condition of the phone, which is a significant difference.
You'll still fetch a "decent" price for the phone, but the condition of it will both decrease the price of the device, generally speaking (it will also open you up to more aggressive haggling if you sell yourself in person, as people will use the percieved damage as a way to attempt to get you to lower the price); as well as decreasing the effective audience of people who would be interested in the device, which could make selling more difficult (or take longer).
Whenever I see a device with dings, dents, deep scratches it makes me concerned about potential non-visible internal damage to the device.
A phone with tons of surface scratches just looks ugly and makes it harder for me to resale when I'm done with it.
There are too many iPhones being sold with very small price variations in the Good and Mint "categories" to settle for a phone with dings and scratches if you're looking for a phone in Good or Mint condition.
For "Fair" devices, you can typically get them cheaper on CL or eBay than on Swappa.
If I were planning to keep the phone myself, basically forever and repurpose it after I upgraded (give to a sign. other, a child or sibling, etc.) then the Jet Black would be less of issue. If you're going to trade it back into the carrier than it doesn't matter. As long as the phone functions, they will take it.