This is the reasoning I use when buying a second hand bat. If you're selling it then you don't want it, if you don't want it then why should I want it? More importantly why should I pay your asking price?
Dunno, see both sides - I mean, I've sold on some very decent sticks. For example, a Kookaburra Kahuna RP that was literally brand new - reason for selling? Well, I broke the original one through the toe and Kook kindly send me the one I sld on as a replacement. It was lovely willow, but teh weight was maybe 2oz lighter than the broken one, which was already on my lighter end...
...that said, I see it as like a car. If the bat has ball marks, 40% has to exit the price immediately!