Christian_SEO:
I would say no, although the market may get smaller as time goes on. The thing about used computers is that while anything that is about 5 years old is considered "obsolete", the truth is that many older machines still have the potential for a long as useful life for either a less-demanding user and application, or some dedicated use as a firewall, web server, Asterisk server, Nagios machine, or something else. With the cost of upgrades so low, it doesn't take much to give an old machine much more resources to work with, such as RAM or a terabyte hard drive.
I agree with Christian regarding uses for older machines, and in fact use some older equipment for exactly such purposes, I think the above comment misses the point of the initial question. While there are certainly uses for older equipment that I may be taking out of front line user service, that is not the same as actually going out and buying old equipment for such uses.
I mean really, look at the laptops available here on techsoup. they are all in the $300-400 range. I just bought my son a new laptop, retail, at Staples. This was a brand new machine with warranty with 3 gigs RAM 259 gig HD fast processor etc., etc. and the latest OS. I paid $322.00 for this machine. If I were looking for a laptop for any purpose for our organization, given the pricing of both, even here on Techsoup, I'm going to go with the new machine.
In the desktop arena, yes, there is a bigger difference in price but again, not that big a difference. And as somebody else has mentioned, you can get factory refurbs at retail stores for not all that much more that TS admin fees and again, with newer, faster, bigger stuff under the hood, in many cases with manufacturer warranties.
So, while your arguments are well made for utilizing older equipment you already have, I don't see the incentive to go out and actually buy old machines.
Kevin