Yes, BASELINES is a Scrabble word!

Letter values in Scrabble US: B 3, A 1, S 1, E 1, L 1, I 1, N 1, E 1, S 1.

Is it a valid word?

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

an imaginary line or standard by which things are measured or compared
the back line bounding each end of a tennis or handball court; when serving the server must not step over this line
the lines a baseball player must follow while running the bases

Also: service line.