No official rule, but we will feed you to a unicorn if you do so.
Note: I'm not a lawyer and all advice here is purely from my thoughts
Let's go legalese! (from http://stackexchange.com/legal)
Subscriber is responsible for all of its activity in connection with the Services and accessing the Network. Any fraudulent, abusive, or otherwise illegal activity or any use of the Services or Content in violation of this Agreement may be grounds for termination of Subscriber’s right to Services or to access the Network. Subscriber may not post or transmit, or cause to be posted or transmitted, any communication or solicitation designed or intended to obtain password, account, or private information from any Network or Service user.
Subscriber is solely responsible for any use of or action taken under Subscriber’s password and accepts full responsibility for all activity conducted through Subscriber’s account and agrees to and hereby releases the Network and Stack Exchange from any and all liability concerning such activity. Subscriber agrees to notify Stack Exchange immediately of any actual or suspected loss, theft, or unauthorized use of Subscriber’s account or password.
Subscriber will indemnify and hold Stack Exchange, its directors, officers and employees, harmless, including costs and attorneys' fees, from any claim or demand made by any third party due to or arising out of Subscriber’s access to the Network, use of the Services, the violation of this Agreement by Subscriber, or the infringement by Subscriber, or any third party using the Subscriber's account, of any intellectual property or other right of any person or entity.
Basically, I see no restriction against selling your account, but I see lots of stuff holding you liable for any actions taken via your account.
Also, remember that rep is a rough measure of how much the community trusts you. You, not your account. The trust system goes boink if someone else uses your account.
The ToS are not transferable
This Agreement is not assignable, transferable or sublicensable by Subscriber except with Stack Exchange’s prior written consent
Update: looking at https://twitter.com/#!/gerbil249/status/204935706907377664, this is sort of different. That is gaming the system/fraud and is explicitly disallowed. You could get yourself suspended for that.