PriceFad is a site which allows consumers to view the price behaviour
I find this site interesting as it show graphically the behaviour of commodities over time (in this case consumer goods).
Although this is for new items (I presume as it does not give info on its data sources), you can infer that second hand items will display a similar time-price curve, albeit from a lower base. This is once again support for the notion that consumers should sell their second hand items as soon as possible, before they depreciate to zero.
The site is interesting but lacks any decent search and browsing facility. The search for ipod did not produce any results, or even suggestions. To get a result the search needs to be an exact product.
Here is an example graph.