I can certainly see people selling their votes if the content is good, and that's not a problem. If someone approaches me with a post that I think is great but hadn't noticed before and offers to pay me for an upvote, I'd certainly consider it. It's a win for them, a win for me, and a win for the community (since the content is good).
By contrast if someone offered me money to upvote crap content, I'd have to decline. It would be awfully short sighted of me to reward crap for a couple bucks now thereby jeopardizing the long-term credibility of Steem.
But, to some degree this problem is self-regulating. The people posting crap content are not likely to have the money to buy my vote. By contrast, the people who post great content would likely have a ton of money to buy my vote, but they wouldn't really need my vote at that point.
So, I don't see this as a long term problem. It will be interesting to see how things play out though.
you could just vote your own post up and cash out, so you would not have to sell your vote, right?