Basically, it's possible for any election on the planet earth to be "bought" in one way or another. So you're saying that elections are always suspect? That BitShares delegates will care more about a potential small bribe than about their longer term financial interest and fiduciary duty to act in the best interests of BitShares? I certainly don't see this as a solid possibility.
Modern election systems are more secure because the branch of the state that supervises election process is not controlled by another branch which is being elected.
With DPoS, I think that we are beyond this.
The Separation of Powers into multiple branches has served an important function, basically to ensure that no one person or group has total control. There is a functional aspect to this also, because each branch is handling different work (e.g. legislative branch as lawmaker, executive as day-to-day operator and enforcer, and judiciary as arbiter of disputes). And in a modern parliamentary system, many of these functions are fused together anyway. But in a trustless DAC community, a lot of these typical functions are moot.
BitShares delegates are elected independently. There is no traditional election cycle, so they can be voted out at any point if they abuse the community's trust. Each of them handles differing functions, depending on their utility, in addition to their core function of verifying blocks. So one could say that it's a single-branched system, but really it is a decentralized (101-branched) structure where no one delegate must answer to any other. The notion of a conspiracy of bribery between a majority of delegates is as far-fetched as a conspiracy between multiple branches of a traditional government. One could just as easily buy of a majority of a nation's legislature and of its highest court in order for them to agree with the executive 100% of the time.
And, unlike those branches of human government, delegates have limited power and misbehavior can be detected. Further, if a corrupt government somehow emerged, it would immediately be competing against an honest fork of itself.
(This is where the expression "go fork yourself" comes from.)
Then market forces would drive the dishonest branch out of business.
If not, then the market approves of the new branch and gets what it demands.