These people are most likely lying, both Microsoft and Sony check regularly for dev units in each company. If there's one missing (as the guy who sold it seems to claim), the companies will know and very heavy sanctions will be imposed upon them. They are also very well tagged, so MS will know which one is missing and the place it's missing.
Also, we are talking about a machine that is not confirmed of existing, these will be even more rare, and there's no way that MS would have let a supposed dev kit of being sold. Developers who are not directly involve with the development of a next-gen game are not even allowed to be in the same room as the machine or even see it (it happened with Vita, now, imagine how strict would be with a console).
As I said, these people are most likely lying. Or basing on very old specs.