Yes, STAMPER is a Scrabble word!

Letter values in Scrabble US: S 1, T 1, A 1, M 3, P 3, E 1, R 1.

Is it a valid word?

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

a workman whose job is to form or cut out by applying a mold or die (either by hand or by operating a stamping machine)
someone who walks with a heavy noisy gait or who stamps on the ground
a power tool that stamps

Also: stamping machine.