Agreed, this is NOT a straight line depreciation. Most of the value of buying something used as opposed to new is the price savings and the integrity of the person/place that you are buying it from. Since most people on this don't know you from a hole in the wall, expecting to get 95% of original value is rediculous. For all we know, you rang up an obscene bill with th vendor in those 3 days and we would then be unable to even activate the phone until the bill is paid. Additionally, you may say there's nothing wrong with it, but for all we know you may have dropped it createing a small crack in the display that will only get worse over time, even though it may not be currently visible.
So yes, as tragic as it may be for you, yes, a significant depreciation is realistic and expected. 3 days, 2 weeks, 30 days, no difference. You may want to start with knocking off about 40% then you may get some interest.