I usually buy my lenses used, since there’s no other way I can get my hands on high end lenses. I’ve only had a problem once, where the guy selling “forgot” to mention that the lens was scratched, but I returned the product and got my money back, minus 10 bucks for the return shipping. Anyway, the moral of the story is that it’s safe to buy online used, just make sure you have detailed pictures from every angle. Also, try only ordering off eBay, because they have great buyer protection.
If the sellers reputation on the site is really good then the chances are it should be fine. Make sure to inquire about how long the person has had it and what exact condition it’s in. I wouldn’t buy anything too expensive 2nd hand stuff, just in case.
I’ve got a friend who bought his first ever entry level DSLR second hand off of eBay last year and it’s been fine so far. No bugs or anything. I think like biggles said, if you look at the sellers reputation and user feedback before buying and make a logical decision, you should be fine. Be sure to check the refund policy too.