If working and tested are seperate, are you saying it can be both untested and working? Then if I buy a untested camera and it doesn't work I can safely send it back because it isn't as described. How's that fraud then?
Normally the wording would be something like: "
It appears to work, the lens rotates, the lever advances, but I don't know if it works well"
Somebody selling second-hand an object that he doesn't know (be it a camera, a clock, a car) cannot check its functionality properly, but will say "for what I see, it seems to work". Then, the rangefinder might be out of tune, the lens can be fogged, the focusing ring will have some problems, the lens will have some play, the camera can have any sort of problem, the gasket might leak, and the seller can be in good faith. You don't expect somebody selling a camera, these days, to actually take pictures with it in all conditions and test it properly.
On the other hand, it's with this kind of "not guaranteed" items that the best bargains are realized.
The information that the camera appears to work is in any case precious. You can ask questions to the seller, for instance, if there are evident scratches on the lens (this is something the uninitiated would not think about checking, and actually would not know how to check properly).
Most people would not be able to actually open the back of the camera. It's not that they want it "both ways" and are trying to cheat the buyer. Most of the times they are honestly ignorant.