This could be restating the obvious but I'll add in a few more just in case.
If it can be used in tradeskilling, it will say so when you examine the item. If it's your trade, keep it, otherwise it goes on the broker. You may not get much for it, but you'll get nothing at all from the npc vendors.
If its a spell or combat art book of any kind, it either gets used by you, put on the broker, or if you are in a guild, placed in the guild bank. It's general practice to place at least some of the adepts you collect in the guild bank for others to use, and of course you are meant to make use of any some one else has placed in there.
If the item says 'offers quest xxx', either 'read' it to pick up the quest or trash it. It will be no trade. It will say 'can be examined to learn more about xxx' if its an L&L item, and will the word 'Collection' somewhere in the text if its a collection items. As the person mentioned above, sell those if you don't use them.
Everything else I can think of has already been mentioned above.