Scratched and fogged lens cover on an iPhone 4. Bought it that way.
Affordable and easy enough repair.
I wanted an iPhone but couldn't justify the 2 year contract at $120 a month or the $723 that an iPhone 5 would cost without a contract. I found my phone on Craigslist, in pretty good shape except that the camera took foggy pictures. Previous owner included the box and the phone company verified it was "clean" so we negotiated a price and I bought it.
In a bit of planned obsolescence, the iPhone camera lens cover extends out of the case by a thousandth of an inch or so, causing it to take most of the wear from being carried without a case or being set down on a nightstand. This results in foggy pictures within a few months of normal use, and very often a trip to the Apple store for an overpriced repair.
Luckily, aftermarket replacement lens covers can be bought online for about $5.00. A special "Pentalobe" screwdriver will also be required, due to the proprietary Apple hardware.
Most places that sell the lens cover also offer a 5 point screwdriver that "kind of" works for a few dollars, but using one of these will leave telltale marks on the screw heads or even strip them out. I found a good one on ifixit.com.
Now I have the phone I wanted for a third of the cost of new, on a prepaid GoPhone plan at $30 a month. Total savings over 2 years: more than I paid for my pickup truck!
Always protect the back of your iPhone. Even a layer of thick tape with a hole for the camera lens will keep the lens cover from taking all the wear.
Consider replacing the battery while you are in there, to save taking it apart again later.
As an avionics technician I'd advise you not to be penny wise and pound foolish and risk your expensive phone with a $3 Chinese screwdriver. If you strip the screw heads they will need to be drilled out and extracted, which is a very dicey and tedious operation on such small hardware. It may not even be possible.