Yes, JUSTER is a Scrabble word!

Letter values in Scrabble US: J 8, U 1, S 1, T 1, E 1, R 1.

Is it a valid word?

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

used especially of what is legally or ethically right or proper or fitting
fair to all parties as dictated by reason and conscience
free from favoritism or self-interest or bias or deception; conforming with established standards or rules
of moral excellence