Do you think random internet users would purchase accounts, when they get no value from it? They can still consume all the content for free, paying only gives them the right to put free labor into curation.

I can see some users pay the fee so they can downvote opponents, but otherwise why would they? And of course they can still pay a higher fee to downvote multiple times, if they are angry enough, which is exactly what you wanted to prevent in the first place...