I routinely make 20,000 a game using this feature. Set your sell amount to 100%, and watch as the money flows in.
Nobody else ever uses it though, so it's almost like free money if you have extra resources.
If I have extra of a resource, I put it into the black market. Then, if I run low, I take some from the black market. In other words, I store my resources in the black market. This means that when people buy resources, they generally buy it from me, so I earn tons of cash. And it's sold at full market value, instead of the half market value for the quick sell button.
Quite fun.
Edit: This is for multiplayer, but I am sure AI buys it too.