You could charge for site subscriptions, but that would grant access to the whole site.
Still.... phpBB 2x has mods out there that will only allow certain users to access certain forums. phpBB 3 is nice, but I actually prefer the older version since there are a heck of a lot more mods available for it. I'm a little biased though having used it for a long time & assisting with ezPortalPlus testing and bug fixing.
osCommerce has a free shopping cart that I *believe* has a subscription feature to it.
Another one is Zen Cart, which has a ton of features. I have that installed on one of my client sites & it seems to work fine. It also is php 5 compatible, which I'm not sure that eCommerce is.
(eCommerce states that it is php 4 compatible and doesn't mention php 5 - at least not on their main pages.)
An inexpensive, but not *free* cart with a lot of features (and a demo available), is Green Cart by POZ Scripts.
If you decide to go with phpBB 2.x and you need help locating mods to do your thing, write me and I'll see if I can help. Every mod I ever used with phpBB was free. You just have to know where to find them.
If you go with phpBB 3, you're on your own.
Hope this helps you a little.
PC