It is easier to say what I would, and have, bought used than to list all of the things I would not buy used. I have never bought a new UPS. I have bought nine used ones, three 1200VA Deltec/NSSI, three APC Smart UPS 1000's and three others. You generally have to replace batteries, but that is straight forward -- although expensive -- so I factor that into what I am willing to pay. Every line conditioner I have owned was bought used -- the price drop from new to used is many times larger than for autos -- and there is virtually nothing that can damage a line conditioner. I buy power supplies for TE's used from reputable dealers like Marlin P. Jones and I recently gambled and bought two dozen fans pulled from new equipment. That's about it so far as used is concerned in my opinion.
I overlooked one major item. I like the old HP LaserJet Series II printers and have four at the moment -- all bought used from the Sandia National Labs. I checked the on board counter (among other things) and only bought ones that hadclearly been an ornament in someones office with a couple of thousand pages total usage. I found a half dozen like that in mint condition, paid less than $50 each for them at the time I bought them, and expect the ones I kept for my own use to outlive me. The nice thing about buying printers that were just ornaments, they always have the maximum amount of memory possible, multiple paper trays, all of the manuals, all slots filled with special font cartridges -- plus extras in many cases. Two of them had been configured for networking with control cards that had never been connected on line even. Used in this case translates into "hardly ever used"