Stating that one only wants to get what they themselves paid for it isn't immoral, but it's gauche. Sort of like telling your girlfriend she's almost as good in the sack as your ex-wife.

Honesty sometimes must give way to tact, and there's such a thing as over-sharing.

Obviously the value of a used component depends on a lot of things. A very in-demand piece will often fetch far more than the 50% that most seem to get. For example, if you ever find a pair of Salk's on sale at Agon you can bet you'll pay a pretty hefty percentage of the new cost for them. After all, they're hand made with very long lead times, and few seem to ever resell them. The Naim Nait integrated also sold for 85% of new price on the used market for about a year after TAS gave them a rave review, as did the NAD C320BEE. On the other hand, a used pair of Infinity Alpha speakers won't excite anyone too much...there's tons of them on the market.
I think it's a bit unrealistic to get everything you paid back in all cases. If you bought it used, when you sell it it's even more used. As in the example of the used car, it now has even more miles on it and probably is worth less. The used gear was probably shipped an extra time, with another chance for dings, etc. And while the 2nd owner might get the benefit of the original warranty, a 3rd owner might not. And there's something to be said for the "chain of custody"- if I'm the second owner, and the original owner is someone I trust, I can claim to know a bit about how a piece was treated. As the gear gets passed around more, less can credibly be said about it's history and condition.
Lastly, the price a piece brings used reflects the risk-to-reward ratio it carries. If I want a Shanling CD player, and you want 85% of the new price for your used one, where does that fall on the scale? It's probably out of warranty, and would likely have to be returned to China if it breaks. The few hundred dollars is probably not worth the risk, at least to me, vs a unit with warranty. But offer it at 1/2 retail and I'm more willing to risk it.