why did Celebrimbor allow Sauron?

[Hakon]

I am not sure about the other elves but I am sure that Celebimbor was that greedy for knowledge. I think he was jealous of Feanor's achievement of the Silmarils, and he would have accepted any help possible.

[Galin]

Also, it wasn't only Celebrimbor who had dealings with him there. It has been mentioned above that Galadriel suspected Annatar, but it really does beg the question of why, if she knew his true nature, she allowed him to remain and have his will with the rest of the Noldor.

By the way, this is a good question, especially considering that Galadriel and Celeborn represented (in this version) the initial power in Eregion, and had actually established the realm.

Keeping in mind that Concerning Galadriel and Celeborn is a short and hasty outline, very roughly composed, I think Tolkien had a problem here: he wanted Galadriel to be perceptive enough to scorn Sauron -- but why was he then accepted in Eregion where Gil-galad (who was to become the grandson of her brother, incidentally) had shut out Sauron himself?

For the second edition JRRT not only added that Celebrimbor was descended from Feanor, but that he was Lord of Eregion (and the greatest of their craftsman). While some might think this can mean Celebrimbor ultimately became Lord of Eregion, in keeping with Concerning Galadriel and Celeborn, I rather think that Tolkien abandoned this earlier notion -- or in any event, the published addition need not agree with ideas from an earlier, rapid outline.

So I think it would be Celebrimbor the Feanorian who would decide to receive Annatar in Eregion, and if I am correct it would have been interesting to hear of Galadriel's reaction to Sauron (in a phase before the 'unstained Galadriel' idea appears to have kicked in).