Multiplayer is integrated in singleplayer in such a way that singleplayer gamers can use the AH without the normally tedious steps of first getting into a multiplayer section of the game. In this way they'll use the AH as just that: a singleplayer AH. This is the first game with such an integration which is why my hypothesis is what it is.
I apologize if I missed your earlier explanation but I'm curious why you think the RMAH will be heavily influenced by people just going through the story campaign. They have both the RMAH and the gold AH available to them as well, I would posit they would be just as likely to use either (or none). Also the people just sort of playing for the story probably aren't as likely to do multiple replays in the same frequency as people who are playing for the MP and even if they did I'm not sure that would necessarily have any bearing on AH activity.
I agree with Krovven, I just don't see it. It's not really tedious to get into the MP side of things either, Blizzard made it very accessible and exposed well in the interface. They don't really need items as training wheels or anything, you gradually build up to that through the difficulty progression.
Playing: Fallout 4, Warhammer Vermintide Watching: Into the Badlands, Ash vs Evil Dead