IMO...there are all sorts of considerations...when determining value of used computers.
Installation/recovery media, warranty, terms of sale, etc...all should be considered.
An example of some basic items, http://www.kpsurplus.com/dell-dimension-4600-pentium-4-2-8ghz-512mb-ddr-memory-57826.html
Personally, I would only buy used components (not a system) from someone I had some respect for and some feel for integrity. People often think that items which they no longer have use for are saleable, regardless of condition and possible problems.