Is it illegal for me to only consider women for a position I'm about to advertise for?

Maybe. There are both state and federal employment laws to consider.

The federal laws are pretty clear. Title VII calls for a "bona fide occupational qualification", and what you've described is not one. If it were, anyone could skirt the law by saying something like, my target demographic is executives, and no executive takes a woman seriously. The good news is, Title VII only kicks in at 15 employees. So, if you have less than that, discriminate away.

State laws vary from, well, state to state. Sometimes they add some new discrimination categories (sexual orientation, for example), sometimes they are the same but kick in with fewer employees.

That said, your logic for seeking only a woman is flawed. One could just as easily say that only a man would know how to market to women since he has an outside perspective and can objectively look at what women want. Actually, they used to say that all the time...

Don't know - call me jaded, but I believe reducing potential hires based on their sex is shortsighted unless there is a true "Bona Fide Occupational Qualification" (BFOQ) in place.

Really, how do you know that the candidate doesn't know the market, have experience, etc. to be a valuable contributor to the effort? Tons of males work in the cosmetics industry. Tons of females work in construction trade. Can you be so sure that because of their sex they will not understand or be successful? To say so unequivocally is absurd. It is the person, their skills and experiences that should be reviewed, not what is below the belt.

You may have to be prepared to explain why a man can't do the job. Would a trans-gender person qualify? Are all of your sales people going to be women? If not, no justification to hiring only female managers. Here is something interesting: http://blogs.findlaw.com/free_enterprise/2009/04/can-men-be-hooters-girls-when-can-businesses-hire-only-women.html

If this is correct, you may have a legal problem. Keep it to yourself and hope no males apply.

The safe thing to do would be not to explicitly state that you will only consider women for the position, but rather to word the job description in such a way that it naturally resonates more with women, if such a thing is possible. Maybe a woman could write the job description as a starting point?

Also it is worth reflecting on your preconceived notions that a woman candidate would a priori outperform a man candidate.

I hate discrimination, but depending on the state you are in, if you are under certain number of full time employees (in Massachusetts, where I am, I heard it is 5), certain non-discrimination laws do not apply.