If they work and were taken good care of, then who cares if they are used?
Here's the problem: How will you know this? Just because a seller says so, doesn't mean much...
If you have a power supply, you don't know if it is calibrated, whether the output matches what the readout says, etc. I bought a couple of ECI power supplies from Apache Reclamation (triple output: 1 variable 0-25VDC 1.25A, 1 fixed 5V 1A, 1 fixed AC output, I forget the specs); I paid $25.00 each for them (over the weekend I picked up the "matching" frequency counter for $25.00) - they were in really rough shape, but they seemed to work. I managed to get the service manual from ECI, so I can calibrate them myself.
You may or may not be able to do the same with another brand of used power supply; sometimes, even if you can get the service manual, you'll spend a lot for it...
Power supplies are one thing, though - test equipment is a whole 'nother ball-o-wax. For o-scopes and meters, I would -not- purchase over Ebay, and I would really shy away from "used", unless I was allowed to power it up, and run some simple tests with the built-in test signal. Especially in the case of an o-scope, you would want to get familiar with how to do this, and how to read the scope, so you know what you are looking at is right. This is especially true in the case of an analog scope.
I got my Tektronix 2213 off a guy on Craigslist; he said he had it calibrated by the factory, and gave me everything to prove it, allowed me to power it up and test it. It passed perfectly, with the minor exception of the implosion shield being fogged (found another to replace it, though). If you test a scope, and you know what you are looking at, and you feel that something just doesn't seem right, follow your gut and keep looking; if you have the manual, there is usually a section that details how to calibrate it yourself, but you need a special piece of equipment called a "calibration signal generator" (which needs to be in calibration itself, of course!); these are anything -but- cheap. So if the scope doesn't seem right, pass it up.
For all of these reasons, it is generally best, if you can afford it, to purchase test bench items new - unless you know what you are looking at, and know how to test before you buy (and the seller will allow you to test). I am not saying you can't find good deals on the used market, but you have to be careful, and examine everything thorougly before you plunk your money down, or you could be looking at throwing good money after bad...
