You're not wrong that it's a discount off new, but it's like a used car.
Stock putter loses a lot of its value the moment you take it out of the store. Just for context (not saying it's the right context) - PGA value guide says a trade-in value for it is sub $100.
This is how people wind up with so many clubs in their garage!
My sense is a lot of people on here are either looking for a unique putter or willing to pay full freight when it's only a little more for something brand new.
The adjustable height thing is also interesting, but a little weird, that might also be throwing people off a bit.
Just for context, I found your post cruising the forum for a new blade putter (I'm currently experimenting with an old wilson alignment putter that I slapped a new grip on and some lead tape on the bottom - early results are promising for a blade!).