We are currently selling access to 'Premium' groups on our site by creating marketpress products for each premium group.
It's clumsy though because the only way we can do it is by making the group private (so people can't join automatically). That's not ideal because we actually want the group to be very public. We just want people to have to pay to join/contribute. But also, the marketpress admin has to manually add users to the groups (using and also having issues with BP Group Control) when payment is received. It would be cool if the user could just purchase access to the group and be added straight away.
I think this feature would be useful for so many buddypress/marketpress installations, especially because groups can be extended so easily. So essentially you are selling premium functionality embedded in buddypress groups.