Hey @SplitReaLityZ!
You’ll want to start with an inventory system, because without the inventory a shop wouldn’t matter, so first things first!
This is something you’ll likely have to put together yourself, however there is TONS of stuff like this in the learning area of this site!
Here’s a link to an inventory tutorial- start there, and then a shop will be a snap! As far as the rotation, you can get random items from an array and populate the shop with that, on a 20 minute timer. But that comes last.
Hope this helps! 