When you loot an item that is tradable, a prompt will appear asking if you want to trade the item. You can disable this feature via /raffle auto-detect off
If you're in a LFR or LFG PUG, you can raffle off items via chat. People who whisper you and can use the item will automatically be added to the raffle. If you raffle multiple items at the same time, they will need to whisper you with the item link.
When someone starts a raffle, players who can actually use the item are shown a roll window with roll options and a progress bar.