Yep - adding a dialog that explains stuff the first time you're on the trade screen is on my todo list. Definitely wouldn't expect someone to just know.
In regards to the inventory I have some requests if you wouldn't mind looking into these.
Inventory:
Make items stack up to 100,000 units so there isn't issues with clogging your inventory with a lot of different ranked crewmembers. After a certain point I'd say make it say something like 10k for 10,000 or so with the specific amount shown when moused over.
Add the ability to buy items in specific amounts such as a prompt that asks you to fill in the number of items you wish to buy, rather than needing to make 200 clicks for 200 supplies.
Add the ability to 'max purchase' where it will give you either as many of a specific unit as are in stock, or until you can no longer afford them.
Fleet:
Add a 'Sort' button like is found in the inventory. Possibly with multiple options like 'Sort by type' or 'Sort by expense' or what have you.
Bump up 'Destroyed' 'Scrapped' 'Repaired' ships to the top of the stack of the fleet when a battle ends. When you have a lot of ships it's a bit of a pain scrolling down a lot.