The Official FAQ states "...if the card checks to see if it was trashed (such as Pixie), it was not, but if the card does not check (such as Tragic Hero), it will function normally...".
My question is some more examples of 'does not check'. I am currently playing the Tragic Hero from the Trash 1) I keep the Tragic Hero in the Trash 2) Do not 'trash' the Tragic Hero (as it is already there) 3) But I still 'gain a Treasure'
Correct. Tragic Hero says "Trash this and gain a card". The basic rule is you follow as much of the directions as possible. So, the directions tell you to trash the card. You can't, since it's already in the trash, so that instruction can't be followed. Then it says to gain a Treasure. Except for the highly improbable case where every single treasure pile is empty, you can follow that, so you gain a Treasure. Acting Troupe and Farmers' Market also work with Necromancer when in the trash. Feast also works with Necromancer if you include removed cards
Pre-2019 errata, Death Cart, Pillage, and Embargo worked with Necromancer. Post-2019 errata, Death Cart can work with Necromancer, but only if you trash another card from your hand