I'm concerned about sensor dust. What if the previous owner was reckless or unlucky and didn't take care to minimize dust getting into the camera? Does it matter for video? Will such dust/detritus show up in the video you shoot?
Thank you for any help!
I have no problem buying used. Just remember, the outside of the camera represents the treatment and will reflect the innards.
Yes, you will absolutely notice sensor dust on a video. You'll notice dead pixels. There are some ways around this, but dust happens and isn't always the fault of the owner. It will happen to you when you buy a new camera.
It's very easy, if slightly risky, to clean your sensor properly. I've done it on all my cameras. It's also possible to reduce the dead pixels (and many have a way to compensate for them, making them invisible). I've seen dead pixels on broadcast tv before, and it's highly distracting once you notice it the first time. It can also be fixed in post, if you can make a map of them.
The biggest issue to buying a working used camera is not dust. It's batteries and cables, and possibly mistreatment of the sensor outside of dust. Batteries are often neglected or 3rd party, or even counterfeits that look original for popular cameras like the LP-E6. If the camera works when you get it, it'll probably be fine, but the batteries need to be run a couple cycles before you should trust them.
If you do buy used locally, TAKE A LAPTOP. Tell the owner to charge the batteries and bring all the cables and software. Download the software to your laptop just in case, and bring your own usb cable. If the owner shows up with depleted batteries, walk away. Do not ever hand over money until you know that electronics work, without a guarantee that they work (like on ebay).
If you need to, bring someone who knows the particular camera well. Take a few pictures with the camera, in raw if possible, and look at it on the laptop. Check the LCD for damage and if pixels are working... but the LCD isn't as important as the final image. Shoot something bright (like the sky) and overexpose it, and look for dead pixels on the lcd. Shoot something dark (like the inside of the lens cap) in manual focus to look for stuck pixels at both fast (1/500) and slow (1 sec). And look at all of these images on the laptop, enlarged. Scroll the pic around to find any low level pixel issues. To check for sensor dust, set to manual focus and defocus camera, set aperture to maximum (smallest), and overexpose a blank white wall or paper by a stop. Look at the pic on laptop. If there are large dark blotches, the sensor is dirty and probably needs a professional cleaning (which can cost between $25-100 if you don't know how to properly do it yourself). If there are tiny OOF specks, that's just a little dust that can be blown off and is normal.
Do not touch the sensor, pretty much EVER. The only thing that can touch it is a cleaning wand made for the purpose, or air from a hand-squeezed blower. If I'm selling a camera and anyone touches the sensor, they just bought the camera.
(BTW, most cameras will have a few dead/stuck pixels, so that's not a deal breaker if your software or the camera can fix it.)