Vorkosigan

Let's try it this way. There is nothing in individual-subjectivism that compels a person NOT to take advantage of another where they can do so, and practical-ought dictates taking advantage of another where it is useful to do so.

Well yes, but your postulate has a corollary: "where it is useful to do so." The subjectivist might reply "But it is usually not useful to do so!"

If I can gain by taking advantage of others, and have no (contingent) moral reason not to, then why not do so?

Why do you need a contingent moral reason? Are threat of police, courts and social ostracization not enough?

You can't give me a reason not to take advantage of others,

I could give you long lists of reasons not to take advantage of others. But you don't seem to be interested in those. Instead, you keep asking me for some reason that exists outside of our conversation. You speak of reason, but you want compulsion. Like I said, you still seek the ideal of absolute morals even when you reject their possibility. Subjectivism means that There is no authority that exists outside of our converstation that I can cite to compel you to obey my moral beliefs.

I can give you many reasons not to take advantage of others. But I can't compel you not to. As a subjectivist, I gave up moral compulsion when I gave up gods and other fairy tales. I can only negotiate, or turn to duly constituted force, or some other socially-supported sanction when you behave in socially detrimental ways.

I have sufficient reason to do so,

If you feel you have sufficient reasons to do so, how can I talk you out of it? Are you willing to discuss? Your argument seems to run "I've made up my mind to screw X....compel me not to." Well, I can't. I can only talk to you, or oppose you. At this point its seems only a shooting war can stop you.

....the reasons for taking advantage of others outweigh the reasons against, so I should go for it, and you have no grounds for saying that I should not.

I have lots of grounds for saying you should not, but again, I have no way to compel you. But you appear to have made up your mind not to negotiate on the matter, so I can only hope you don't get caught. Because a reasonable subjectivist, knowing full well the limits of human knowledge, can never be certain that she's not going to get caught.

Michael