As I have no experience or expertise with the relevant source materials (such as ancient Hindu and Buddhist texts, languages, and archaeology), I have no answer to give. Until I have a good reason to believe otherwise, I assume the consensus of experts (that he existed) is correct. But how would we know? By testing different hypotheses against the surviving evidence and seeing which one best explains all the evidence we have. It is still possible that even after this we will have an inconclusive result and thus won't know the truth, because the evidence needed to decide the matter has been lost. But in general, does the non-existence of Buddha explain the evidence even as well as a historical Buddha does? You would have to ask an expert in the relevant historical period.
Was Buddha a real person? How do we know?
The Atheist Perspective
Why do seemingly logical individuals arrive at such differing conclusions regarding existence, all the while maintaining that logic allows nothing else? Is the same information being evaluated? Is the same definition of logic being considered?
Answers: Atheist, Christian
Why would your book refer to Jesus as "the Messiah" but only in passing as if it is just a nickname? Why is this major "controversy" not addressed and discussed in the Quran "in detail" as it should have been as per 27:76, 16
Who is Jesus in Islam and how does he compare to Mohammed?
Where in the Qur'an are Muslims told to pray five times a day?
Apologeticspress.org and CARM.org disagree on whether baptism is needed for salvation. Both are Christian and both quote the Bible in support. Who is right and why?
Atheist God Is Not Great: How Religion Poisons Everything Christopher Hitchens Atheist Why I Am Not a Christian: Four Conclusive Reasons to Reject the Faith Richard Carrier Atheist Humanism: A Very Short Introduction Stephen Law Atheist Atheism: A Philosophical Justification Michael Martin Atheist Hitler Homer Bible Christ: The Historical Papers of Richard Carrier 1995-2013 Richard Carrier Atheist Sense and Goodness Without God: A Defense of Metaphysical Naturalism Richard Carrier Atheist The God Delusion Richard Dawkins Atheist Not the Impossible Faith Richard Carrier