At what population level does the Trade Depot or Global Trade HQ open and what are they good for?

Every City has it's own Trade Depot and Global Trade HQ for selling and buying items with other SimCity BuildIt Mayors from around the world.

Trade Depot:

The Trade Depot is located just outside your city entrance in a factory style building on the left-side and it's the place where you can sell your excess inventory, either from your factories or from your shops.

This is the first of the 2 exchanges that opens and the Trade Depot becomes active when you reach a level of 8,000 Sim Population.

Everybody gets 8 trading slots and can advertise a new slot every 5 minutes on the Global Trade HQ which makes it visible to a huge audience so if the item is reasonably priced and in high demand (e.g. Expansion or Storage materials) then you can make several thousand Simoleans in under 5 minutes. 

You can expand the number of trading slots at a cost of 10 SimCash each to increase capacity for selling and making more Simoleans.

Global Trade HQ:

The Global Trade HQ is also located just outside your city entrance but to the right-side and in a taller building with the letters HQ written at the top. This is the place where you can buy any items being sold in other City's Trade Depots from around the world and it opens once you reach a level of 10,000 Sim Population.

Overall it is a simple system and when you click on the Global Trade HQ building it opens up a window full of fast moving items for sale.

 

The minimum sale price is 1 Simolean and the maximum sale price depends on the item and is set by the  trade system so there can be a range of the same items on sale for completely different prices.

Speed is the name of the game when trying to buy an item since there are also many other Mayor's trying to purchase the same thing so for the most popular items it can be near impossible to purchase as some prices and certain busy times of the day.

However as you get to know the trading system you get a feel for what is easier to buy, for what price and what is not worth the time or effort regardless of price.