I think the Grand Exchange will have more of an impact of the RuneScape Forums than it will on Merchanting. I think it's safe to say that 60%+ of the RS Forum population is in the Market Place, and when the Grand Exhcange comes out..it could possibly be like an In-game Forum MarketPlace.
I'm not as worried about merchanting as I am about the Forums. I think it's safe to say 90% of the Forum use is in the Market Place, and the Grand Exchange would be the equivlilant of an in-game Market Place Forum. :shame: