EDIT TWO: On the first test, I determined that PMD does NOT have an autosave after missions, and turning off the game after completing a mission but before saving the game will cause you to lose all data past the last save point.
On the second test, I completed a mission, saved the game, then checked the Kecleon Shop(Left side) and took note of the items. I then turned off the game's power, turned it back on, and checked the left side of the Kecleon Shop again. The items were the same.
Thus, I can conclude that the shop's merchandise is determined the moment a new day starts in the game, and soft resetting to change the item selection is impossible without hacking.
As for the dungeon Kecleon Shops, soft resetting would be impossible because the dungeon's layout is randomly determined the moment you enter it, and the shop's items are determined also.
The tests were carried out on my copy of Pokemon Mystery Dungeon: Explorers of Sky.
EDIT: Searched a ton of sources, can't find anything on soft-resetting the shop. Guess I'll just test it myself, after I get back from karate.
As a side note, it appears that the normal button combo used to soft reset will not work in Pokemon Mystery Dungeon. You have to use the power button.
I believe that the Keleon Shop's items are determined the moment a new day starts in the game. The shop's item selection is partially predetermined by what point you are in in the game. For example, it will not hold Keys until you have unlocked the post-game,
The exact items in the shop appear to be randomly determined, so theoretically, you could soft-reset. However, the game auto saves when you fail a mission by calling for rescue, then canceling the rescue request and opting to just lose your money and items instead of have someone rescue you, which makes soft-resetting impossible. I'm not sure if it auto saves after every mission, failed or not.