I've bought from KEH and eBay many times. I've had all good experiences for the most part. I do not hesitate to deal with KEH, I'm selective when dealing with eBay. Knight Camera, Shutterblade seem to be reliable, professional re-sellers. Have been impressed with Unicphoto offerings, but they are a bit pricier, have very good feedback. Have also bought from private individuals with good results, but consider this a bit riskier.
I recently bought a very nice-looking FE2 from KEH via the auction site. It was stated in the description that the camera was actually the one in the multiple photos. It was stated that this was a very nice, BGN grade body. When I received the camera, it was not the same one as the photos. Had substantially more brassing, but everything else seemed in good shape. I contacted KEH, and received an email back stating that when being examined just before shipping, some substantial defects were found in the offered camera. They selected the best looking, BGN grade camera they had in stock to ship. While I was somewhat disappointed and suspicious, I turned down their offer for a refund, and kept the camera, and decided to give them the benefit of the doubt based on all my previous positive dealings with them. I have found no deficiencies in the camera they sent.
I purchased a Yashica D from Knight Camera, but when I received it the shutter did not work. I ended up sending it in for a CLA. Had some difficulties getting a response from Knight Camera when I contacted them, but eventually made contact and they offered to reimburse me for half the cost of the CLA, which I was happy with.
I have also bought one used lens from Adorama. When I got it the zoom mechanism was not smooth, would "catch" in a certain range. I contacted Adorama, they refunded my money and paid for the return shipping, no further questions asked and apologized for the inconvenience. I wouldn't hesitate to deal with them again.
My position is that we are dealing with very old, used equipment. Sometimes things get by on an initial exam. As long as the seller stands behind their offerings, I am happy.