What's to stop someone from selling their reputation?
How about common sense...
The points don't matter
Why pay money for something that you could otherwise get for free?
The only real value that rep has beyond opening features on the site is a sense that the community trusts you a little more (not much, but a little) and an ego boost. Neither of the latter two reasons would be met with purchased rep.
More or less purchasing rep would be a bit like intentionally purchasing botched plastic surgery, you would be laughed at by the community you were trying to impress.
And don't think people wouldn't find out, with 2.2 million users on SO alone the chances of posting a rep sale anywhere on the net and getting away with it are pretty slim.
Migrated from comments, because I thought it was a good question.
What about the privileges? 10k to one person may not be the best
trade, but 50 rep to 200 people each looks like a great deal for 200
spammers, and you don't even lose the rep you're selling.
– Jan Dvorak
You would face a basic black market marketing issue: How do you at the same time advertise that you're in business and remain hidden? Selling smaller amounts of rep to more customers means more exposure and a higher risk of getting caught, which eventually leads to a higher price as the risk reward ratio gets skewed, but customers aren't likely to be willing to pay a higher price, as rep is otherwise freely available.
Some market forces still apply in a black market.