Can I automatically sell junk?
Unfortunately, no. The inventory system is quite frustrating and you have to sell everything manually. It does not even properly categorize stuff!
Luckily for us PC rpg gamers, there are a few great mods who address these issues.
On Witcher Nexus in the user interface mods section, there's a mod for better hud icons and another for better inventory. It adds a new category for misc (junk) items, ability to sort by name or cost/weight and so on.
Also included are some nice changes to alchemy so the game won't waste your quest items on your next potion.