If someone converts to Islam simply for the sake of marrying a Muslim, and this person is not from the people of the book, is the marriage Islamically legitimate?
Walaikumussalam w w
First of all we should not doubt the intentions of a revert. We should trust them and help them to learn more about our beautiful Deen.
Even if there was some doubt, the marriage would still be valid, unless they deny being a Muslim. Denial is kufr and in that case the Nikah will be nullified even if he was from the people of the book.