Oh boy, this is gonna be a long thread lol!
Basic selling systems for selling/dynamic stock prices for selling items would fit perfectly in Zone Idle. Each merchant would have different prices at different times (e.g. Merchant no. 1 could buy the bolt at XX rubles while Merchant no. 2 could buy the bolt at YY rubles). Nothing fits waiting more than waiting for the most optional prices.
On top of this, merchants have variable inventories which are considerably more valuable (AND LIMITED) purchases available. Of course, this means MORE WAITING to get your desired purchase!