It's true that the DAO hacker could sell off into the market to deflate the price, but this outcome from all the permutations of what they could do is not very likely. Although we don't know the personality of the DAO hacker, we can probably make these reasonable assumptions:
1). The hacker believes code is law (because they did what they did).
2). The hacker probably resents the Ethereum foundation for breaking the rules and confiscating their code is law earned ether.
3). If the hacker liquidates they will probably have to reveal their identity (or at the very least risk revealing it). It is definitely not in the attacker's interest to reveal their identity.
4). Even if the hackers's sole motive is profit, liquidating into the market is not the only way to make money in this situation. The hacker is in a great position to manipulate the price of ETC. It has already been suggested by others that the hacker could fund development efforts in the ETC community, which would have the knock on effect of inflating the price.
Knowing aspects of the hacker's personality would sure help us to figure out what their next move might be. In a nutshell though, it is mainly a consideration of how much risk the hacker is willing to take revealing their identity. A lot of different things can happen, but if I was the DAO hacker it sure would sound appealing to become a God-like mystical driving force that supported the development of the ETC ecosystem.