1.) Negatives for all sellers/buyers of accounts is potentially not effective for targetting account farmers:

That would mean everyone gets hit.... big sellers, small sellers, escrows, the guy who once sold 1 account.  The people who farm huge tons of accounts are mostly posting on accounts which mean nothing anyway.  More so than the person who once sold 1 account etc, or the person who escrows.

You might aswell give negatives to all people who whore themselves out to use signatures to promote business - probably another bad idea.

2.) charging people to use signatures function via a donation to the forum:

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.  Problem is people will just spam and then since they are a paying customer, who will stop them?