A few years ago, universities were a good resource.  You might see if
there's a recycle shop at the university, or St. Thomas.  Usually, they
are under contract and have to periodically replace (think quota) their
inventory.  There's also the Minneapolis freegeek org or something that
accepts used equipment, refurbishes.
Tom

On Sun, 2010-09-12 at 16:55 -0500, Jason Hsu, embedded engineer, Linux
user wrote: 
> I have two very old desktop computers (early-2000s vintage), and I am considering replacing one of them with a somewhat newer desktop computer (mid-2000s vintage).
> 
> What stores sell used computers?
>