Yes, MARGINS is a Scrabble word!

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

Is it a valid word?

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

the boundary line or the area immediately inside the boundary
an amount beyond the minimum necessary
the amount of collateral a customer deposits with a broker when borrowing from the broker to buy securities
(finance) the net sales minus the cost of goods and services sold
the blank space that surrounds the text on a page
a permissible difference; allowing some freedom to move within limits

Also: security deposit, gross profit, gross profit margin.