I think the good 'stuff' - i.e. top condition, provenance, rarity or all combined - sells as or even more quickly than 4-5 years back. Most of the NOS with B&P don't even make it onto the dealer's websites or CM anymore. BUT: I think - at the same time - collectors have become more discerning and more educated (not least through forums liks this and through experts sharing their knowledge) and the 'bad stuff' sells less easily. Maybe 4-5 years ago a watch in average or below condition was still sold - now there are watches where price doesn't even matter anymore (collectors simply don't want low quality anymore). So, even a 30 or 50pc discount to 'good condition' doesn't sell you the watch. So, maybe some of these watches you see on the CM for a long-time have an obvious issue (re-dialed, re-cased etc. etc.) that now people spot - and the seller simply hopes one day somebody less informed will come and pick it up.
Anyway, just a theory - but I am confident that the demand and hence price spread of top-quality versus average/below is augmenting dramatically. And 'normal' or 'doubtful' simply doesn't sell anymore - collectors are more sophisticated.