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
If the book of Genesis is not to be taken literally, why did god send himself in human form to die in order to save us from a symbolic sin committed by metaphorical characters (Adam and Eve) who never existed?
Can justice be done using punishment of infinite nature for a transgression of finite nature?
Answers: Atheist, Christian
Do Muslims believe in reincarnation?
Answers: Muslim, Muslim
If creation happened about 5000 years ago, then how come a supernova was seen in 1987, but it was located 170,000 lightyears away?
If God said "Thou shalt not kill" then why did he flood the whole world leaving only Noah and his family alive, and how is the destruction of Sodom justified (killing people for engaging in "immoral" sex), is God exempted from morality?
Atheist Humanism: A Very Short Introduction Stephen Law Atheist Atheism: A Philosophical Justification Michael Martin Atheist Why I Am Not a Christian: Four Conclusive Reasons to Reject the Faith Richard Carrier 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 Not the Impossible Faith Richard Carrier Atheist The End of Faith: Religion, Terror, and the Future of Reason Sam Harris Atheist Proving History: Bayes's Theorem and the Quest for the Historical Jesus Richard Carrier