If the RMAH is up from day one then from day one there will be people using it. I think especially from day one you will have all the people looking to make D3 a career and a job trying to hook eager credit card players in to a sh!tty trade. In addition to looking for stupid people you will have the people who played the beta and have some idea about what a good/great item is trying to ride the first wave and cash in quickly.
Given how fast some people could make it to 99 in D2 I have no doubt that within 2 weeks you will have top tier items showing up on the RMAH, some people are that good or just that committed to spending 24h/day on the game.
As I mentioned in another thread I believe that the RMAH will divide the player community, which I don't think anyone feels is that much of a good idea, though I'm sure there are many people who don't give a rat's @$$ either way.
As to preventing illegal selling my suggestion is that it's not possible. Somehow, in some way you will have 3rd party sites, the trick is not attacking them directly but providing a sanctioned alternative that is safer, easy and fair. While the RMAH fulfills 2 of those criteria it's certainly not fair as people with deeper pockets win every time, nothing to do with skill or any other intrinsic attribute. An auction house should be purely gold based and as such give more importance to the in game currency.
And to those people who support the RMAH trying to get people who don't to convince themselves they are wrong I say this - it's not my job to do your work for you, dissent is not a sin and it's not unpatriotic. Have a nice day.