Yes, TRAILER is a Scrabble word!

Letter values in Scrabble US: T 1, R 1, A 1, I 1, L 1, E 1, R 1.

Is it a valid word?

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

someone who takes more time than necessary; someone who lags behind
an advertisement consisting of short scenes from a motion picture that will appear in the near future
a large transport conveyance designed to be pulled by a truck or tractor
a wheeled vehicle that can be pulled by a car or truck and is equipped for occupancy

Also: house trailer.