From how I'm reading what you wrote, it sounds like you either made a grammatical mistake or are contradicting yourself. As such, I'll just state how it generally is. I'm absolutely sure there is more nuance, but the general concept is correct.
In short, Sega was not authorized to sell the dev kits. The dev kits are the legal property of Nintendo. Functionally, this situation would be similar to the selling of stolen goods. That is, the person selling the goods does not have ownership or authorization to sell them. Local jurisdiction, and facts specific to an individual case, may change how this plays out in court on an individual level. In general, the legal owner is entitled to the return of their property. That means, usually, that the buyer would have to relinquish possession. At that point, the buyer would have a case against the person that sold them the goods. Situationally, that could mean the buyer is out of luck. They may be able to go after the seller to recoup their losses, but there is no guarantee they'll be able to recover it.
With regards to the buyer's obligations to validate the purchase, they're not obligated to verify the sale is completely legit. They are, however, still subject to the consequences of the purchase. As such, it's in their best interest to at least make some attempt to verify. For example, I wouldn't expect just anyone to know that dev kits aren't supposed to be sold. However, if you're a person regularly involved in those kinds of sales it might be something you should know.