Many smaller ISPs get their own CIDR block allocations from an upstream provider. The route advertisements of those smaller ISPs, in turn, are aggregated with those of their upstream providers. IANA delegated to NSI the responsibility to allocate IP address assignments for all of North America. ARIN could change that. It won't sell IP numbers, but it will charge to recover its costs to allocate CIDR blocks and to determine that any requested allocation is justified.
Membership currently includes the privilege of attending semi-annual meetings.
Although smaller ISPs will be able to purchase small blocks directly from ARIN, the prices they'll pay are liable to be higher than what their upstream providers will charge them for the same address space. Uncle Sam, in the person of the National Science Foundation, has determined to stop footing the bill in the U.S., so, like it or not, some kind of fee-based mechanism is going to replace the free ride sometime in the immediate future. Perhaps so, perhaps not. As of now, it is the only proposed alternative to taxpayer funding-and taxpayer funding is definitely the wave of the past.