This.
The autotrader etc all show the asking prices and as some of them (the cars that I have an interest in) have been for sale for months (and in some cases, many months), that shows that that particular price is too dear.
Ebay is useful in showing trends for selling prices and at the moment, a lot of the recent generation of VLM (very large Mercs!) seem to be moving very slowly (W220's, C215's etc) where they have large engines.
The W140's seem to be moving quickly and the R129's very slowly. No doubt the time of year, but query whether the rapidly lowering prices of the R230 is having a pushdown effect.
There is the "don't build them like they used to" but if you have, for example, a late R129 and an early R230, there is a certain appeal in being able to press a button and the roof folds away (or back up) unlike the 'take off the hardtop, store said hardtop' routine of the R129.