It is unlikely that there are any devices which resemble a Kindle enough to pass as a fake at least for now. I'm sure all the various cloners out there will be busy making eBook reader knockoffs called the Krindle soon enough but they won't look quite right if you know what a real one looks like.
As with most things, anything that is too good to be true generally has some hidden strings attached. The cheap Kindle being advertised is most probably stolen, broken or was purchased with a stolen account or credit card. There is the remote possibility that they did get given one and don't want it but can't return it so they're selling it but I'd say those cases are rare.
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