Eurorack modules aren't made in great numbers for the most part. There are a couple of really successful modules out there that are the exception.
The prices stay high because of limited availability. "Out of Stock" is pretty normal on most dealers' websites.
I typically avoid used modules because you just don't know who's been messing with them and if all of the features work on the day you receive it and try to test it. Saving 20% isn't enough of an incentive to risk my 80% on a dud module and all the hassles that come along with it. Dealers will simply exchange a defective new module. A one-time sale from a third party is a crap-shoot.