The Global Trade Link (GTL) is where other players will list their assets for selling if they do not wish to do announcements in the Trade Chat and direct negotiations and trades with other players.
You can always access this system, but it's only possible to make purchases or listings after you get the 4th badge in your first region with your character.
You can access this system through the game menu (or or pressing P if you are playing on PC) or through the computer in a Pokemon Center.
Assets purchased through this system while opening it through the menu or P will be sent to your mail; If you open it through the computer, items bought will be sent directly to your bag and Pokemons will be sent to your team if you have slots available for them, otherwise they will be sent to your mail.
The GTL also has a search and advanced search systems which you can use to find exactly what you are looking for without the need to browse the pages.
To make your listings, you need to access the system through the computer in a Pokemon Center, go to the tab "Create Listing" and add the asset to be sold and its price per unit.
Be aware of the listing fee. It changes according to the price per unit, with the max fee being $25,000. You can confirm the fee before creating your listing.
It is highly recommended that you verify the assets being sold and price before confirming the listing. Trades made on the GTL cannot be reverted or refunded.