Yeah I'll second this. You can also use the @extend directive to make selector definitions inheritable (kinda), in the case where 'rounded' and 'large' do have to be classes.

  .buy {
    @extend .large;
    @extend .rounded;
  }

I should never have to use .large and .rounded in my HTML directly, but I might use both a 'buy' and a more specific 'buy-special-case' which itself @extends 'buy'.