It loads a list from our database of all items with the allowed rarity range, and lets you specify what bottom limit you want on the profitability of your items, or you can load your own manual list like always.
For the timing, this is what I have right now: It stays Main Shop ABing or Igloo'ing until the correct minute rolls around, and then it starts refreshing at whatever second past the minute you set it to (I have it so you can set from :XX to :XX seconds past the minute NST in the settings). The NST timer is extremely accurate, since I don't let the program do the counting, I let the OS do it. All the program computes are deltas between the server time and the local system time to map the OS time to NST. I'm not sure how important super accurate timing is, because I'm still not entirely convinced that it restocks right at :53 or :54.
I will also have the option to set a custom wait time for the Attic specifically, since we know approximately when it will fill up (seems to be :53 or :54 past the minute according to my tests, but the damn thing restocks so little that I had to wait overnight to get this data and it's not even conclusive). So you can set it to refresh extremely quickly in there during the time you specify (this will be restricted for non-advanced members).