Yes, CONVERTIBLES is a Scrabble word!

Letter values in Scrabble US: C 3, O 1, N 1, V 4, E 1, R 1, T 1, I 1, B 3, L 1, E 1, S 1.

Is it a valid word?

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

a car that has top that can be folded or removed
a corporate security (usually bonds or preferred stock) that can be exchanged for another form of security (usually common stock)
a sofa that can be converted into a bed

Also: convertible security, sofa bed.