They will run a diagnostic on it and if it passes you are generally good to go as long as it not dented or cracked anywhere. Last year MM reduced my trade it for £660 to £149 for a cosmetic defect. I had them send it back to me and they had a sticker on it pointing to the defect in the titanium frame... the defect was the plastic antenna strip... they just didn't what to honour the price they gave. Apple Store walk in and they passed it for £490 okay it was less than £660 but by this time the price had dropped a lot and it was better that £149. so this year I'm just gonna take my current iPhone into the Apple Store and trade it in for £500 and I'm happy with that its so hassle free and they refund it on to your pre-order just like they would if you posted it to them.Hopefully so. It’s just a couple of marks about the size of a pin head however as the colour is silver under the blue you can see them.