Help iPhone 7 plus Jet black owners?

Well, i bought iPhone 7 plus 2 weeks ago and since then i kept it in flip cover as apple mentioned that its easily scratchable. And today i removed its cover and saw from backside's upper and bottom side (antenna area's) shine is gone and there's some thing which cannot be cleaned and easily visible and not looking nice. I don't know what to call it but there's some dirt i guess or it looks as if paint is removed so my question is how to get and maintain the shine of jet black color? I like to keep my phone clean and never had any problem before with any phone. I feel like I shouldn't have chosen this color because this happens in 2 weeks (even in its case) and what will happen in near future and no one will buy it when i try to sell it to buy another phone even if someone will buy but they won't give me good price. Sorry for my bad grammar since my language is not English i tried my best to explain it to you and now expect you to respect me and give me a better solution. Kindly reply and, Thanks in advance.

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