Yes, TRANSPOSING is a Scrabble word!

Letter values in Scrabble US: T 1, R 1, A 1, N 1, S 1, P 3, O 1, S 1, I 1, N 1, G 2.

Is it a valid word?

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

change the order or arrangement of
transfer from one place or period to another
cause to change places
transfer a quantity from one side of an equation to the other side reversing its sign, in order to maintain equality
put (a piece of music) into another key
exchange positions without a change in value
change key