What is the ruling on money paid in exchange for relinquishing ownership of an Islamic website to someone who wishes to endow it as a waqf? Does this affect the reward of the original work? And what should the owner of the website do regarding those who participated in its building, whose work cannot be accurately estimated or who cannot be reached?

It is permissible to sell the website and benefit from its price, unless you had intended to make it a waqf (endowment) for Allah; in that case, it is not permissible to sell it. Your sale of the website does not invalidate your previous work nor does it negate its reward. For those who worked with you seeking reward from Allah, nothing is obligatory for them in return for their work, but it is better to pay them something to please them.