What special markup options are there in Discussions and how to use them?
Solution 1:
YouTube Videos
Paste a link to a YouTube video and it will replace the link with a box containing the video title and thumbnail. Clicking the box opens an embedded version of the video.
http://www.youtube.com/watch?v=idt3HoPz9io
User Content
Add a link to any screenshots, artwork, videos, Workshop items, collections, guides, or Greenlight items and they will be given a preview. Clicking will pop up a preview.
Screenshot: http://steamcommunity.com/sharedfiles/filedetails/?id=117886695
Artwork: http://steamcommunity.com/sharedfiles/filedetails/?id=163988748
Videos: http://steamcommunity.com/sharedfiles/filedetails/?id=178460483
Workshop items: http://steamcommunity.com/sharedfiles/filedetails/?id=159186571
Collections: http://steamcommunity.com/sharedfiles/filedetails/?id=95744629
Guides: http://steamcommunity.com/sharedfiles/filedetails/?id=159458915
Greenlight items: http://steamcommunity.com/sharedfiles/filedetails/?id=93777661
Inventory Items
Right click an item in your inventory and click Copy Link Location. Pasting that link in a discussions post (or even a chat window).
http://steamcommunity.com/id/keavon/inventory/#440_2_1464041478
Trade Offer Buttons
A button to send a trade offer can be created by pasting a link to your public trade offers found under Item Inventory » Trade Offers » Who can send me Trade Offers?
http://steamcommunity.com/tradeoffer/new/?partner=88225129&token=0FwymFEe
Emoticons
I'm not sure if this counts, but you can type the code for an emoticon which will appear in your post provided that it exists in your inventory.
:tdealwithit: 
I'm sure there's more that I can't think of right now. I will update this if I find any more.
Solution 2:
To do this simply go in you inventory, right click an item and click 'Copy link address' then post it in your topic/comment :)
Solution 3:
Simply go to your inventory, right click on an item, choose copy link and paste it into the post as is.
You can get your public trade offer link by going to your inventory, trade offers and then "Who can send me Trade Offers?". You'll see your own link down towards the bottom under "Third-Party Sites".
