Here's an idea to encourage more votes to be bought, will turn it over to the economic automation mavens for possible implementation:
Create a Bayes-type partnership that, if a vote is put on sale within the first three days of a proposal's voting period but not bought by someone else within the first six, then it buys it and/or casts its own votes, specifying a random value and/or the opposite of the seller's default. Alternatively, just have it cast its own votes in weighted proportion to bribes transferred to it for no other purpose. Would obviously rely on keeping it funded, but some players have been willing to keep Bayes funded, so... (Tying it to one or both banks would keep things interesting.)
