Yes, BUCKWHEAT is a Scrabble word!

Letter values in Scrabble US: B 3, U 1, C 3, K 5, W 4, H 4, E 1, A 1, T 1.

Is it a valid word?

BUCKWHEAT is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a member of the genus Fagopyrum; annual Asian plant with clusters of small pinkish white flowers and small edible triangular seeds which are used whole or ground into flour
grain ground into flour

Also: polygonum fagopyrum, fagopyrum esculentum.