Yes, YARDER is a Scrabble word!

Letter values in Scrabble US: Y 4, A 1, R 1, D 2, E 1, R 1.

Is it a valid word?

YARDER is a valid word in the official Scrabble dictionary (US) and the UK Scrabble Dictionary, but not in Words with Friends (ENABLE).

(used only in combinations) the height or length of something in yards
a winch (or system of winches) powered by an engine and used to haul logs from a stump to a landing or to a skid road

Also: yard donkey.