Yes, CASTERS is a Scrabble word!

Letter values in Scrabble US: C 3, A 1, S 1, T 1, E 1, R 1, S 1.

Is it a valid word?

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

a worker who casts molten metal into finished products
a shaker with a perforated top for sprinkling powdered sugar
a pivoting roller attached to the bottom of furniture or trucks or portable machines to make them movable