I have turned down buying gear when people say "I just want what I paid for it"..... it irritates me that someone has used (caused wear) in an item and then wants all thier money back. I Always sell items for less than I personally paid, unless I've had something years and the value has increased dramatically.
Even then I'd ask the person to make an offer rather than set the price myself.
In regards to WIS, then this particular illness sees people flipping watches, sometimes within hours of having bought it, so in that situation I accept that it is reasonable to want "what you paid for it". Still best to phrase the advert differently though.