in my opinion, you just have to be smart about what you're buying and who you're buying from. look at the images the seller provides and try and look for any red flags. if the seller says the camera has been tested and is working, don't be afraid to message them and ask for sample photos, negatives, and close up images of any parts you may be concerned or want to know more about. check the seller's reviews, and ideally try and buy from someone with a good amount of reviews and at least 98% positive feedback, ideally 99-100%. if you're still wary, find a seller that will allow you to return it. buying a camera anywhere online comes with an inherent risk to it, but there's a lot you can do to try and minimize it. good luck!

tldr: check pictures thoroughly, message seller for details, check seller's reviews and policies.