Am looking to get a used iphone 5s off ebay. There are some power sellers on eBay selling refurbed grade B/C phones within my price range. However the housing tends to have some dints/scratches on it. I have seen screens and back casing selling for about £35. Is it easy enough to change the this over ? Is it time consuming ? Are they reliable casing ? i have seen seen instructions and Youtube videos but its not a simple matter of just popping off the case and screwing in a new one.