Use the notification center (the bell in the page header).
There are several types of notifications:
Notifications of website updates;
Click the bell at the top of the page to open the Notifications section:
I saw a notification that someone wants to buy my item, how do I make a trade?
Open the notification center by clicking on the bell icon in the page header. Install CS.MONEY extension and sign in with your Steam account.
Create a trade offer and confirm it in the Steam mobile app:
Wait for the trade confirmation from a buyer. After that — the deal is completed: