Obviously there must be some judgement (the seller, no visible flaws).
But yet, is there a significant risk of buying something which will break in the next few weeks buying an used camera?
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The risk is in the seller being dishonest about something that is already wrong with the camera. If it is good working order, then there is no reason for it to suddenly break within weeks of buying. Equipment does break down, so it is always possible that it will coincidentally develop problems.
If buying from a dealer, ask for a 3 month warranty period (or longer). If you can physically inspect the camera, do. Take along an SD card and shoot some images and review them. Borrow the camera for a short period for a "test drive". If buying from eBay or similar, go by the seller's reputation/feedback and description of the product. Ask questions, does it have any problems, so that if it does you have some recourse because they misled you.
There are no guarantees when buying used, that said, there are things you can do to increase your odds. Ebay is the last place I consider when buying. Not that I would never buy camera gear from there, it's just harder to know if someone is legit, and since a lot of non-camera people sell camera gear, it's harder to get a good sense of condition than from someone more familiar with camera gear specifically.
That said, private forums are where I do most of my used gear shopping. Good photography-centric forums will have a buy/sell forum and will have some way to track a seller (and buyer's) reputation.
Keep in mind, no place is entirely safe from fraud, so you still have to do your homework and take your chances. That said, a person might not sell to you if you have no reputation, though having a verified paypal account can help there.
And using paypal is pretty much your only protection. I have purchased a Canon 40D and a few lenses from these private forums with no problems, but your mileage may vary.
Also, it can suck, but with craiglist, you can see the item in person before you purchase it. Something as complex as a camera body may be difficult to fully evaluate in person in a short amount of time, but lenses are easier to tell if they work or not by simply trying them.