I mean how long could it take to provide an interface to trade items? The interface isn't even particularly good.
Well, they have to integrate into Pay Pal and other money channels, so it probably wasn't 2 days of coding and testing. That takes time out of the project schedule period. So lets say they used 15% of their project schedule for RMAH, that took away from the core game.
I suppose its certainly possible. But the trading part of items and integrating with paypal could certainly be done by someone who was not designing the core game and I would find it surprising if the same folks were working on that piece. And 15% of a game the size of Diablo 3 to simply integrate with a payment system seems a bit of a high estimate. I've actually done that work myself as a developer and it's not particularly difficult.