Mike - the ones you see on the MBZponton site are usually being sold by long-term owners, who feel they've got a gold mine (or a total basket case).
Pontons are selling pretty quick on BaT and Hemmings recently.
I sold my 220S on the MBZponton site in less than two weeks. For the 190b, MBCNA actually read my blog of the coast to coast trip, then made me an offer to buy it and show it around the USA as an example of how long these cars last. The car wasn't perfect, but the only work I needed on my 7000 mile trip was a tightening of the fan belt - I didnt' bring the right size wrench in my travel kit!
Very cool cars - especially the Hydrak system.