CCBill have support for download store type operations, I started looking at it a few years back but stopped when MessMaster opened the UMD's download store as that was easier. Oddly enough I was re-reading over their setup documentation the other day.
You need to be or know a decent programmer to make it work but the basic principles appear to be:
You pass a product ID back to CCBill in the "Buy it now" button code.
They pass that, and the buyer's details (incuding a username, same as for a membership) back to you once they've processed the transaction.
Scripting at your end is then responsible for creating the user their own directory with the content they've bought in it and the user/pass for them to access it set up. So they get their own tiny little members area with just the stuff they've bought in it. Access remains in place for a short while (say 5 days) then you auto-delete it.
Like I said I never got round to actually implementing it so don't know if there's complexity under there I've not spotted, but for anyone used to working with interactive websites it should be reasonably easy to set up.
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