I'm trying to expand my knowledge of items. I saw someone playing my main (Evelynn) and running a dark seal, which i've never done before. Also, why don't they ever upgrade it to Meija's?
Another item that caught my eye is Poacher's Dirk. What champs/roles build this and in what situations?
None, it has a camp cd on it. If you were playing a heavy invading jug and the cd was removed it might actually be viable.
Poacher's dirk should only been thought of as a purchase if the enemy jungler is AFK, and you need it for an item, otherwise never buy it Dark Seal is something taken by champions for the 25% bonus healing from potions, but a few champions, like LeBlanc or Katarina will take it with refillable, sell it later if they don't do too great, and build it into mejais if they get the early kills they need
Quick large Jg kills will give you benefits. Better for late game when nobody cares for jg as much. Dark seal is for pot sustaining literally.
Dark Seal is a good early game item bought first or second back when you're also running potions of some kind, usually corrupting pot, for extra sustain and power. As an added benefit it can also grant you AP if you start getting kills so it can be nice if you expect to get ahead.
Poacher's Dirk is good on AD assassin champions who plan to do a lot of jungle invades.
well 
is taken when you are a fast clear champion who can safely invade or can speed things up with 
to compensate. maybe like 
(mainly for raptors on kayn) though I would NOT do that with a champion like 
around,who have more kill potential then let's say an 
and 
is taken for 2 reasons IMO
A:to stack it for AP in a lane you know you can safely get kills in or can sorta abuse their kit to use(
being a main offender...) SOMETIMES turned into 
,but it is such a feast or famine item,and unreliable as you lose up to 50 AP and the movement bonus turning it into what is basically a 1400G 
and B:sustain. one of the passives is 25% more healing from potions. which is not unique so you can stack the things,some people ( ...I do him sometimes) grab 
rather then something like 
as they have some other way to get mana,or don't need it 
sometimes 
so rather then getting the last hit passive they go for HP healing.
an odd case of this is 
. as he gets CS with his Q so he goes behind the enemy turret and CS..es. there where no minions can take it. and with all that damage he get's one,two upwards to THREE 's for 25%-75% more healing,some AP and mana. which makes for a lot if you have with two 125 charges or 
with three. the goal being to sustain the hit's to get guaranteed CS but also not be 10 HP when somebody tries to get you
