1.) Negatives for all sellers/buyers of accounts is potentially not effective for targetting account farmers:
That is not a negative, that is a positive. You should not be selling accounts in the first place.
2 BTC was mentioned, someone said that was crazy high, i agree its high. You could charge on a monthly basis 0.1 or 0.05.
Your suggestion is absurdly foolish. Charging a monthly basis of that amount, which is lower than what some earn in 1 week, will do nothing at all.
Problem is people will just spam and then since they are a paying customer, who will stop them?
Once there is no incentive to spam (paying signature campaign), you will be surprised at how quickly most of them reduce their posting.
There have been periods where I have seen 100+ permabans in the modlog.
This forum needs more of those periods.