What is wrong in Sankara's accepting from Buddhism?
Buddha rejected and hated the Vedas, Buddhists given a chance will persecute all Brahmins. Shankara himself criticized Buddha in his Brahma Sutra Bhashya because Buddhism is completely at odds with Hinduism. Buddhists do not believe that Brahman exists and do not accept it as the substratum of the world. They do not believe in an absolute reality.
"No further special discussion is required. From whatever points of view the Buddhist systems are tested with regard to their plausibility, they cave in on all sides, like the walls of a well dug in sandy soil. [Buddhist philosophy] has, in fact, no foundation whatever to rest upon, and thus it is foolish to adopt it as a guide in the practical concerns of life. Moreover, the Buddha,3 by presenting three mutually contradictory systems of philosophy — teaching respectively the reality of the external world, the reality of consciousness-only, and general emptiness — has himself made it clear either that he was a man given to making incoherent assertions, or else that hatred of all beings moved him to propound absurd doctrines that would thoroughly confuse all who might take him seriously. Thus, the Buddha's doctrine must be entirely disregarded by all those who have a regard for their own happiness."
- Shankara, Brahmasutra-Bhashya
Hindus reject Buddha and Buddhist philosophy.
Is not Lord Buddha an avatara of Lord Vishnu?
Puranas are not an authority on anything and they can be interpreted in anyway one wants to interpret it.
What if Mayavada and Buddhism are the same. Is it not all Lord's 'leela'? Did not Lord Buddha come to re-establish 'dharma' (Sambhavami yuge-yuge)? New theories would not be able to weaken advaita and Hinduism.
If he had come to re-establish dharma he wouldn't have discarded the Vedas and the Vedic rituals.