Hey Thomas,
thanks for bringing this up!
It has become such a problem for Steem lately, so EOS needs a solid plan to prevent this. I'd love to hear more insights about how we can achieve this.
I guess one positive is, if vote-selling is forbidden by the constitution, at least it cannot happen via EOS tokens or in any other public manner (As it is happening so enourmously with vote-selling bots on Steem right now..). So this would prevent the BPs from receiving paid votes from any large number of token holders.
However, due to the pareto distribution, one will likely only need a small number of influential votes, to be voted in as a witness. So this is where the preventive efforts need to focus the most. Large sums being paid to few a large investors.
On the upside, the constitution adds an important psychological barrier for potential participants in such a deal, as their funds could be frozen, if vote-selling was to be proven.
It will hopefully become the norm, that BPs disclose their financial balance and new investments. (I think EOS New York has already done so ..)
Also paying for a significant number of votes will raise the spending side of BPs substantially, and therefore make them less competitive (at least in theory).
In general, I see self-voting and vote-selling (which are basically the same thing), as the biggest problem for dPoS. It basically morphes into PoS, where the platform simply rewards the highest stakes. The more you have, the more you get. As far as I can see this would amplify the pareto principle exponentially, which effectively means we have created somewhat of a ponzi-scheme.
Once mafia-like structures are established on the platform, they are likely irreversible.
It is a hugely interesting social experiment, whether strong identity validation and governance can sufficiently outlaw such behaviors.
Humans will always be inclined to act in short-term self-interest over the long-term group interest, so some degree of vote-selling will inevitably happen.
While it won't be possible to eliminate those behaviours completely, it may be reduced sufficiently, so that trust in the system as a whole is not broken.