Yes, ETA is a Scrabble word!

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

Is it a valid word?

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

a terrorist organization organized in 1959 by student activists who were dissatisfied with the moderate nationalism of the traditional Basque party; want to create an independent homeland in Spain's western Pyrenees
the 7th letter of the Greek alphabet

Also: basque homeland and freedom, basque fatherland and liberty, euskadi ta askatasuna.