you can do whatever you'd like but i see two potential problems which would steer me clear of this idea:

i'm wondering how well tumbled marble will take a sealant and how often you'd have to reapply.
and, grout on a counter top is one thing i'll never do again.

might be just me but this sounds like upkeep i wouldn't want to deal with. good luck, i hope everything turns out better than you expect!

it's not a good idea.

I agree. It'll take a sealant fine. The problem is it won't completely stop the staining. Also, with tumbled stone, you can't have small grout joints, being that the stone is notorious for bad sizing. That's going to make for some tough cleaning.