Lowest risk:

Adorama.com
Bhphotovideo.com
keh.com

All three are good sources. Long established with great reputations. They offer a 14 day right of return (no questions asked) and between 90 days and one year store warranty (the term varies) for any failure of the gear. They also tend to be pretty conservative with their condition ratings. I've bought used stuff from all three that looks good as new, but was rated one or two stops lower.

mpb.com is a new source that's starting to attract a lot of attention. I haven't used them yet.

Depending upon what system you are using, also check for refurbished. Canon USA now sells factory refurbs directly from their website (if you see Canon refurbs elsewhere, they are not "factory" refurbs). Nikon is starting to do that too, though some of their refurbs are still being sold by their biggest dealers. Check the details carefully... Canon offers free shipping on orders over $100, but charges sales tax (around 7.5%, I think). And Canon warrant their refurbs for a year, same as new. Nikon only warrants theirs for 90 days, same as used (I don't know about their shipping and/or sales tax).

When you buy "factory" refurbs, it's a manufacturer's warranty, instead of a store warranty like when you buy used. Aside from Canon and Nikon, I don't know about other camera and lens manufacturers, but it would be worth exploring if they offer refurbs. Finally, shop around because refurbs aren't always the best deal. I was all set to buy two cameras that way a couple years ago, but found out that between sale prices being offered and the bundling some retailers were doing, I could actually buy new for a bit less. Still, refurbs are worth checking out.

EDIT: Someone mentioned the lens and camera rental companies. I'd forgotten them and have purchased through them, too. In fact, Lensrentals gave me great deals on a couple brand new cameras and a few accessories some years ago, when no one else had them available yet or, if they did, were marking them up well above MSRP due to the scarcity. I'd be a little concerned buying previous rental gear... it doesn't get treated particularly well by the users and spends an inordinate amount of time bouncing around in the back of big, brown trucks getting to and from renters. OTOH, the rental companies are pretty diligent about maintenance... so their stuff may look like hell, but work great! And I sort of prefer gear that looks "well loved".... it might be less appealing to thieves. Also makes me look more like a pro! )

I've bought used through Amazon.com, too... but you have to be more careful and it's riskier. While it's on Amazon, if it's not direct, who knows who the sellers are, if there will be any warranty or even if there is if it would ever be of any help, if needed. I've seen misinformation and somewhat questionable stuff there, too. "Store warranty" in a listing means "gray market" and in many cases "no warranty" as far as I'm concerned! Supposedly Amazon provides some warranty and confidence ratings for anyone selling there... but I've never tested it (thankfully) and don't know the details.

eBay is certainly a possibility too... but even riskier. Even highly rated sellers there might have no real working knowledge of specific pieces of camera gear or know how to properly test it. Most sellers are honest... but even if they accept returns, in the event of a problem you can still end up out of some money for shipping and insurance, as well as the time and hassle. eBay promises a warranty of sorts, but I don't put much stock in it. Seems a long, drawn out process that doesn't always work out well. I'd basically assume there's no warranty on anything bought via eBay (and I've bought hundreds... no, make that thousands of items there... I've also sold a lot).

Paypal can help insulate you to some extent on eBay.... but only so much and not all sellers offer it. Many other sources use Paypal now, too.

Watch out for shipping costs and import duties on gear bought via eBay, too. Today there are a lot of things in the US are offered with free shipping. But if you buy internationally, the cost of shipping can be substantial and varies a lot by country. I've noticed that the UK and Japan seem to have the highest shipping costs... while it costs relatively little from Russia and some of the former SSRs, China and other Asian countries. I once bought 20 cameras in a lot from Scotland... cost me $2 per camera to purchase, but $4 per camera to ship them! And that was economy (took 4 or 5 weeks and the box arrived looking more like a soccer ball!) Thankfully they were "military" cameras designed for rough duty and most survived the trip (and sold for around $40 each, when offered individually rather than in a lot).

Your local Craigslist.org might be a good source, too... though it's also rather risky. Be sure to only meet sellers in safe places (a bank lobby, police station, busy coffee shop, etc.) Also be prepared. Study prices beforehand and learn how the gear works so you can properly inspect and test it. You may need to bring along a camera and computer to test a lens, for example. Also bring a small flashlight to inspect inside lenses. But don't be concerned if you see a few specks of dust in there.... that's not a problem, Doesn't effect images and there's always some since lenses aren't assembled in dust free environments like clean rooms. You sort of need to know what you're looking at and looking for, to buy off Craigslist. There are no warranties (even if one is promised, assume it means nothing).

Don't overlook local camera stores. Some of the better ones have good used equipment dept.

I've actually had good luck at local secondhand stores too.... but it's mostly been vintage gear. You never know what you might find, but when you do find something the low prices can be fantastic. I paid all of $24 for a really ugly (plaid!) camera bag from a local store once... inside was a viewfinder and a 100mm lens for a medium format camera, along with a Gossen Lunapro light meter and a bunch of other stuff. I kept a couple things and sold the rest on eBay for $400+ total! Another time at the same store I paid around $30 for a plastic case with a like new, black body Nikon FM2n, two zoom lenses and a flash inside. I also have a Tamron 90mm macro lens that I got there, also like new, and paid all of $20 for.

But those deals are rare. And I've passed on a lot more after closer inspection (such as a digital bridge camera with a big, fast Leitz lens, but when I looked closely the lens was shedding coatings inside... or a 24mm Nikkor lens that caught my eye, but my flashlight showed fungus growing inside).

Roberts Camera, National Camera Exchange, Samy's, Henry's (Canada), Glazer's, Pacific Rim, Kenmore Camera, Hunt's, Beach Photo... I've bought from them all and others at one time or another. But again, a lot of it was vintage collectables. I mostly go to B&H, Adorama and KEH for my modern, daily user gear... and Craigslist for local purchases.

Not true... . Not as complex as a camera.... and pretty darned reliable... but "$*#@ happens!"

I've had AF, aperture and image stabilization systems fail on lenses.... Two of them were premium Canon L-series, too. None of them were ever dropped, though they all had fairly "high mileage" and weren't "babied". The L-series were worth repairing, others weren't.

Old mechanical lenses didn't live forever, either. One of the first I ever bought (1978?)... a Konica 57mm f/1.2 Hexanon... literally fell apart after 10 or 15 years use!

In the end, lenses are pretty reliable, risk buying used is reasonably low so long as you can closely inspect and test a lens. Besides, unless you make your living with them it's not all that critical if there's a failure anyway. But if a lens works well when you test it and looks to be in pretty good condition, likely it will have a long, happy life.