I used to work the surplus sales while going to school (graduated in 04), and unless things have changed the computers they sell are pretty horrible. The people I remember buying them would buy them by the pallet just to get memory out of them. They tend to be rejects from computer labs and staff offices. The nicer stuff gets recycled around the departments untill they are outdated enough to send to surplus.
I always thought the best items to get from surplus were desks, file cabinets and shelves. They may be dirty or kind of rough, but they're heavy duty.