I just bought my third used DSLR. No problems so far.
I buy from a local store, so I can check out the kit before commiting, and get a couple months warranty. It can also be cheaper than the auction sites sometimes.
I ask for the number of actuations, too. On Nikon, this is can be found out by taking a picture and looking at the EXIF data. Canon is not consistent about this. In my experience, there are many lightly used consumer and prosumer cameras.
Nikon pushes you more to the used marker because of the lack of new low end models with a built-in motor, and before that, because of the lack of "affordable" full frame.
What you'll probably get is some dust in the sensor. If you ask, the store may clean it once for free.
You should only buy a body if it comes with all the important accessories: battery, charger, nonstandard cables. Verify first if you can download the manual. If it's not included, you may have to pay some bucks for it.
Don't forget about the bundled software, if you care about it.
If you don't mind walking around with a big brand sign, check that the strap is also there.
Things like LCD cover, eyecup cover, disposable battery adapter are usually missing.
If you buy a lot of missing bits, it won't be worth it, if there's an equivalent new option.