1. Damage per second. It takes the base attack and the attack speed of the weapon into account (as well as any additional damage)
2. Near the blacksmith in Haven is a modify weapons station, you can unequip and equip weapon mods like grips and handles.
3. I'd sell weapons that have lower DPS than the weapons on your characters. As for valuables, I sell all figurines and gems but DO NOT sell the creature research drops. As a rule of thumb, I sell the things in the valuables tab that do not have a discription or do not have a yellow ICON.
That's the second time you've been just a sec faster now. I gotta learn to type faster.
