Many products go through surpluses and shortages, the two you mentioned both tend to be in shortage more often than not because Carbon as a mining product is limited to premium players and having limited availability in each country. Electric Car Motors similarly can reach big shortages because they require Metadium as a supply product and it can only be sourced from Tiny Atlas world and sold manually on the markets on the core 5 worlds.
Take a look at the current supply and demand for each on your world, and assume that orders tend to be fulfilled in a 'first come first served' manner to plan out your ordering.
For example on White Giant, Carbon currently has a supply of ~ 21M units and demand of ~ 314M units of Carbon. So if you placed an order now, you would reasonably expect it to be fulfilled in 314M/21M -> 15 game months. So if you adjust the low level marks and order quantities of your corporations that use Carbon to