Originally Posted by twiztidsinz
1). Can the shop appear on a map you've already been to?
I've heard yes, it can... just like rare spawns can show after you've cleared a map (I've seen this happen and had two chests on the same map).
2). If so, is there a minimum time you need to wait (like 15 minutes for the shops items to change), or can you just keep going in/out of the screen to refresh?
No. And I am pretty certain about this. The Bakery is considered its own separate room, and rooms do not change. Also, that "refresh time" is, as far as I know, store only. If you get a rare spawn by backtracking you were always going to get a rare spawn by backtracking in that map. It wouldn't matter if you did it 15 minutes, 2 days, or even 4 years after you started the map. Its just a random occurrence the game chooses at the beginning of the map.
In any case, the bakery doesn't behave that way anyways, it is its own separate room, You can even revisit it if you haven't left that map yet. Just remember to buy the spears taht are shop specific there.
Also, although I do think that a Pizza guide would be useful, you do not get a quest or unit for finding that, thats why the bakery is so much more important.