Is it safe to buy a used PSU? What are some precautions to take?

Considering buying one and I just wanted to know what I should look out for as far as used units? What to avoid, and if I should even but a used PSU at all.

Are you talking a psu from someone local that you could test or one from online like ebay?

I have purchase used PSU before but form people I knew in from Tech forums that I felt okay buying them. I normally stick to new otherwise and with good warranty like the EVGA Supernova 10 year warranty but that is me. Right now using an old PSU that I purchase in 2006 and still kicking.

Online Loche. Not to many local listings in my town.

I agree with ITangel. If you are buying used from a known source I would say you are good to go. I would not buy from someplace like ebay. Have you like at the EVGA site? They have a b-stock section were you can snag good prices on refurbed psus.

There isn't much you can do really.. I mean take a DMM over and test the rails on load? That seems a bit ridiculous to me (perhaps I am too trusting). Maybe look inside for bulging caps.. see if the chassis was opened or damaged... the basics I guess. Same thing though, make sure its a quality brand/model PSU.

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.

The only PSU that I would buy from eBay or Amazon that are used including local would be a Server PSU. They are meant to take a beating and often will last a long time without any issues and take just about any load that you trow at it.

Don't buy something with even 1 swollen capacitor unless you have decent soldering skills. The same for anything built with non-Japanese capacitors in the low voltage section.

The safest used PSUs may be pulls from new computers where the user wanted to upgrade to a PSU with a higher power rating. Right now, I'm using one made in 1999. It's needed only 1 repair, when it was new and some solder joints at the transformer cracked (shipping damage?).