I agree with that in theory, but I think degree and seller intent are important. (And again I'd stress I'm not talking about legality here, just personal ethics).
Say I bought a Moog DFAM new from Moog for $650, which is about the price brand new through a typical retailer. I decide I don't want it and I sell it as an open-box item for $750 and someone buys it. Fair enough. I'd say that the buyer got kind of a poor deal, as it's not too hard to find a used DFAM in near-mint condition for $500-600 (I paid $480 for mine); but hey, it's certainly no seller's responsibility to offer the lowest or fairest possible price imaginable every time they sell something. People offer to sell things above their average market value all the time, and I don't think they're all doing something bad or unreasonable.
But in terms of personal ethics, to me, there's a range that's conscionable, and that range is based on the seller's knowledge of the generally conceded value of the item he/she is selling. If I take my Moog DFAM and price it at $2500, purely staking my hopes to sell it on the notion that someone will buy it simply not knowing any better ... to me, that's really slimy. If someone does that and can just murmur "buyer beware, buyer beware" to themselves and sleep okay at night, so be it. Personally, I couldn't.
That being said, in all transparency and fairness, I took another closer look at the listing I originally mentioned at the start of the thread as I got to thinking more about this, and it appears to be for a version of the module that's now discontinued, which I didn't initially realize. Combine that with the fact that Make Noise seems to be a hot manufacturer in modular right now, and that puts the listing in a different and much more understandable context, where they're pricing it for hardcore collectors who want something no longer available through the normal channels. I still think it's a greedy mark-up, but since it's an "out of print" unit, so to speak, I've actually no issue w/ it.