Go to your name, upper click it and below will show more tabs, you click "Activity", when the page opens on right side of the screen you have some buttons, search for the "Trade Offers" button, click it then you have in right side again, lowest tab called "Who can send me trade offers?" you click it and scrolling down you'll see a link, that's the trade offer link, to enable trade offer button (on tf2outpost, bazaar.tf or backpack.tf) you go to settings (must be logged in through Steam) and there they ask for your trade offer link, use the classic method copy - paste, when your settings are saved with your link in them, everyone will be able to send you trade offers.

Thanks.