If Advaita existed prior to Buddha, he would not have advocated sunyavada at all because Advaita is the final and ultimate truth.+

It is no use arguing Bhagavan Buddha is wrong or Sage Sankara is right, but where we are going wrong in our understanding of the non-dualistic or Advaitic truth, propagated by the great sages of the past. Some say, that without the sunyavada, Advaita philosophy could not have come into existence because Advaita starts from where sunyavada ends. That is why they say it is an extension of Buddhism. 

If Advaita existed prior to Buddha, he would not have advocated sunyavada at all because Advaita is the final and ultimate truth.

Since the Buddhist and Vedic scriptures have been passed down by hearing, they were written down only relatively late, so one wouldn't know whether to rely on the times they give. Also, a lot depends on the translation. Each 'Sloka' or sutta is open to many layers of interpretation.

Sage Sankara said:~ Talk as much philosophy as you like, worship as many gods as you please, observe ceremonies, and sing devotional hymns, but liberation will never come, even after a hundred aeons, without realizing the Oneness.

Sage Sankara himself had often said that his philosophy was based on Sruti, or revealed scripture.

This may be because Sage Sankara addressed the ordinary man, who finds security in the idea of causality and thus in the idea of God—and Revelation is indispensable to prove the latter. He believed that those of superior intelligence, have no need for this idea of divine causality, and can, therefore, dispense with Sruti and arrive at the truth of Non-Dualism by pure reason.

Sage Sankara’s Supreme Brahman is Nirguna (without the Gunas), Nirakara (formless), Nirvisesha (without attributes), and Akarta (non-agent). He is above all needs and desires.

Sage Sankara says:~ "This Atman is self-evident. This Atman or Self is not established by proofs of the existence of the Self. It is not possible to deny this Atman, for it is the very essence of he who denies it. Atman is the basis of all kinds of knowledge. The Self is within, the Self is without, the Self is before and the Self is behind. The Self is on the right hand, the Self is on the left, the Self is above and the Self is below".

Satyam-Jnanam-Anantam-Anandam, are not separate attributes. They form the very essence of Brahman. Brahman cannot be described because the description implies a distinction. Brahman cannot be distinguished from any other than He.

The objective world-the world of names and forms has no independent existence. The Atman alone has a real existence. The world is only phenomenal. :~Santthosh Kumaar