My only gripe is that there should be a membership level between advanced and premium that excludes selling privileges, since sellers only represent a small percentage of the community.

Sellers should have their own membership level. I have no intention of selling anything, but if I were, I would consider $20 a month a small price to pay, to have my products exposed to such a large, captive audience. If you can't sell $20 a month to cover the cost, you have no business being a seller. That's see more one of the reasons I believe sellers need their own membership level. There should be a membership level that offers what 'Premium' does, less the cost of being a seller.