I agree with some others that account farming is directly related to signature campaigns and spam.

It's a tricky situation.  Freedom of speech and all that...

The easiest solution would be to ban account sales officially.  Mods could be free to start deleting the topics, and perhaps even perma-banning those who engage in the behavior.  This could give those who want to fight the issue some ammunition to do it.

It could be taken a step further by raising the price of registering an account using TOR, restricting the number of accounts that can be created from an IP address, and pruning all accounts that haven't been active in 2 years.  This could make it a little more difficult for the account creators.

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However, my favorite solution is this...

BAN SIGNATURES FOR FORUM MEMBERS WHO HAVE NOT DONATED TO THE FORUM.
Create a new level of donator called the "Supporter" that can be reached with a 2 BTC donation to bitcointalk.
Only Supporters, Donators, & VIPs are then allowed to have signatures.  Perhaps with each level carrying different restrictions.

This would bring in a bit of revenue for the forums from anyone who wants to participate in selling advertising on bitcointalk, and raise the cost of running a signature campaign.  All this while reducing the spam, spammers, and those who fund them.