I have bought many of my PSU's used from ebay (because I often can't afford new, plight of a college student and a low paying job). Never had any of them fail, so far.
Corsair HX520
Corsair HX620
Corsair HX750
Corsair VX550
Corsair CX600M
And four from this forum:
OCZ PSU of some sort, it failed and was unstable from the day I got it. Probably the first used PSU I bought, and the worst one I've ever owned.
PC Power & Cooling Silencer MK III 850W -Review sample from OK Wolf (so wasn't used that much, but it was tested thoroughly), powering my test bench very well for the past 3-4 years.
Corsair RM650i - Review sample from OK Wolf (so wasn't used that much, but it was tested thoroughly)
Corsair CX750M
My main tip is to avoid the ones that the seller says they are unable to test (buy at own risk). They're cheaper, but there is a risk they won't work.
Also avoid modular ones that have very few or none of the cables.
I avoid buying lower-quality/lower rated power supplies used, just because they have a tendency to not last as long. I also avoid buying from unknown brands or really cheap power supplies (don't buy a $20 power supply to power an $800+ PC).
Buy good brands and models of power supplies (some Antec, Corsair, Evga, PC Power & Cooling (if it's one of the better models, as I hear some aren't as good), Seasonic, Superflower, etc).
Maybe my experience is unique, but I've had good luck with buying used power supplies so far.