You beat me to it... I was going to cite Baker Romance books as an example. I can't see how they can hold at current prices over time. I agree with GGA as well. Only the wild bondage/menace covers will maintain value, in my opinion.
I'm actually surprised by the strength of non-key cover pre-code horror right now. I think in the long run, these books will start to fall by the wayside and the keys (BCM 50, WM 5, MM 12, etc.) will keep tracking upward.
Disclaimer - I keep buying a lot of stuff that is either undesirable now or will be in the future, so I have no horse in this race