I decided to snap pictures of the first turn, the tenth turn, and the 12th turn of each Item Shop in the game to see what is there in the inventory and when it changes. It changes at the half way point, so on turn eleven. Not turn ten, but turn eleven
So the best thing anyone could do would be to try to anticipate getting to a shop on the eleventh turn because that would give you the best options. I never thought about it, but it would be interesting to have to Shop Hop Boxes on deck after the halfway point so that you can ensure you get a least a couple of the very best items available. Doing that might ensure 2-3 or maybe even 4 purchases in the latter half of the game. On bigger boards with great items, that's something to consider trying out. Even on these smaller boards I would say that it's probably just as beneficial. But I haven't tried out that strategy yet. Maybe I'll put it to the test one day. Most Mario Party players do not consider planning ahead like that so it would definitely be interesting to see how it would all play out