sometimes it's people just signing up to "drop" the price.
perhaps they aim to lower the market benchmark (or at least give sellers the impression) so that they can buy the second hand equipment at a better price.
in any case, how's this for depreciation of value - i bought my k20d for $1.7K 2 years back, even offering it at $530 = no takers. i think too many people are buying cameras, and new ones at that.... well, the economy's recovering (or has recovered), hasn't it?
It might not be a price drop per se. Some people might "overprice" their stuff so that when somebody bargains they can give a lower price. eg. The market price for a 70-200 f4L with lenscode UV-UW might be 800-850 but you will see people posting anywhere between 750-1000. Those selling at 750 will be sold within minutes since it is underpriced. Those listing at more than 850 will remain unsold for weeks or it might finally be sold for 850 but the seller never update their thread with the final selling price. This gives the impression that there is a huge variation of the second hand price when in actual fact there isn't.
The actual "market price" might be known only to those who monitor b&s long enough and only for items that are traded often.
This I feel is one of the shortcomings of the current b&s, that it isn't really transparent.
I remember last time when anyone can comment in the b&s thread. When something is underpriced you immediately see a comment offering to deal or offering a higher price. When something is overprice, someone will come in and state the "fair" price followed by accusations of "price policing" by the seller. Makes it much easier to find out what is the market price.