wait sorry what?
I take it you’ve never watched the Venture Brothers.
The Numenoreans knew exactly who Sauron was the whole time they were dealing with him. But it was a combination of factors that made the Numenoreans trust Sauron. First of all, Sauron was just that good. Deception was always sort of his strong point, and Tolkien largely attributes his control over the Numenoreans to his strength:
Yet such was the cunning of his mind and mouth, and the strength of his hidden will, that ere three years had passed he had become closest to the secret counsels of the King; for flattery sweet as honey was ever on his tongue, and knowledge he had of many things yet unrevealed to Men.
It’s worth mentioning that Sauron’s flattery was so successful because he was telling the Numenoreans exactly what they wanted to hear. Factor #2 was the already strained relationship between the Numenoreans and the Valar. it’s true that the Valar had given Numenoreans a home a many gifts (and, probably, information about Sauron that should have kept the Numenoreans far far away from him.) But within the first 2,000 years of Numenor’s history, some of the people had grown to be less than grateful towards the Valar. They were jealous of the immortality of the elves, and didn’t understand why they weren’t allowed to sail to Valinor themselves. Over time, this grew to a general dislike for the Valar (and, eventually, the elves as well.) Fast forward to Ar-Pharazon: much of what Sauron was saying was not only flattering to the Numenoreans, but was specifically designed to feed into this bitterness towards the Valar. He said, for example, that the Valar were “seeking to enchain Men in servitude to themselves”, and that the true god (who he said was Melkor/Morgoth) would “make [Men] stronger than they.”
Finally, other than the cultural/historical anti-Valar trend that allowed so many of the Numenoreans to trust Sauron, I think Ar-Pharazon himself was an important factor. He was considered the “greatest and proudest" of Numenor’s kings. Once Sauron won him over personally, Ar-Pharazon’s popularity and influence over his people made it easy for most Numenoreans to extend their trust to Sauron as well.
Of course, don’t forget that not all the Numenoreans trusted Sauron. The Faithful (or what few remained so late in Numenor’s history) new better, and it was their distrust of Sauron and Ar-Pharazon that enabled any of the Numenoreans to escape the island’s destruction.
SOURCES: The Silmarillion