You can ask your companion to sell your junk items for you. Press N to bring up your crew page, then press the button (on the right side of the active companion) to sell all junk items . Or, alternatively, right click on your companion's portrait and select "Sell Trash Items".
When you do this, your companion will leave for 60 seconds with the items, then return with credits. You can do this from anywhere, so assuming you don't need your active companion for a fight you can do other stuff while he's gone. When he returns, he'll be active again (no need to resummon). There are additive per-character Legacy perks, Legacy of Leadership I and II, that reduce this to 30 seconds and 5 seconds respectively.
Your companion will not sell items above "gray" level.
