It seems that the number of D-level speakers offered for sale was (and remains) pretty small (including dealers).
Not sure exactly what evidence stereo5 has for stating "there so many pairs for sale". As far as I see it the volume was small and commensurate with the price-band (very high = not many, very low = lots).
However, stereo5 does make a valid point wrt price/quality -- no-one will ever get that equation fully solved. I was probably in attendance when Mr. stereo5 heard the D5s (there was an open house at Blink). They sounded good, but were we also hearing the mega-high-end Aavik mono blocks and pre-amp which were used also?
Staging and depth was generally terrific but a couple of highly rhythmic tracks did seem to lack a certain "punch" -- but then again adding a pair of JL subs into the mix could be a winning combo (per Jonathan Vallin's recent review).
One observation I have wrt pre-owned availability & numbers ... I certainly have noted a large number of D'Agostino Momemtums for sale (not only in US but also in UK). Do I read anything sinister here or is this an artifact of D'Agostino's market share/dominance? Ditto for Wilson speakers?
Finally: bo1972's comment wrt use of diamond of a load of codswallop - the drivers in D-series have fully diamond coatings, diamond coating over ceramic. This is achieved using a very high Voltage bath which "fires" carbon atoms at exceedingly high velocity [resulting in the impact pressure being exceedingly high]. This technology is well-established in scientific instrument manufacture (i.e., for electronic microscopes) and does take a LOT of energy and a lot of time (ceramic "coating" takes about 5-6 days for a 6in driver and diamond coating adds an extra few days on to of that).