Laning Buy in Order normally, can skip around. I always get Extra Regen first tho, extra ammo and regen is nice in lane.
Mid-Game Buy in order usually. Quicksilver Reload Imbue Sleep Dagger. You can skip Ricochet if your not going to be doing alot of farming only get it if you can tell the game will drag out.
Late Game Can skip around. Get Orange items if want more damage. Green items if want tankiness