The game states that the "Traits" of an Action card are not restrictions neither requirements. That is for example, Troll-Feller Strike can be acquired and used by any player. My question is, does it work the same for actions with traits such as Dark Elf, or Khorne or Tzeentch etc. which are intended for monsters? Can characters spend exp to buy them?

Thank you

Yes and no. There are some "bad guy" type actions that are allowed by players (see dark magic spells as a good example), but the ones you mention are monster action cards and should not be given to players, because these actions generally make up for the lack of detailed equipment and talents which monsters possess.

Thank you JasonRR for the comment, but can you clarify to me if what you say is a house rule or is it stated in any of the books? In that case, can you point to me which book /page?

Thanks!

The monster guide has a list of all action cards for NPC's/Monsters and the players guide has the list of all action cards available for players.

Also you will notice that many monster cards may have effects that just dont make sence as a player. Example: "[comet] Gain 2 agression token" or "[chaos star] loose all expertice token" Also forcing corruption and disease checks that are more long term things that would not make a difference in a single combat, and a large number of cards require you to be "Beast" or "Giant" or "Deamon" or something else.

But again the system is very flexible that way. I guess if a GM wish to allow players to learn an NPC action card on an exception basis, it could be fun for the story as well. I mean if players befiended a beastman herd and stuck around for a while. Maybe they could learn a beastman trick or a shamanistic spell. Unlikely situation, but hell, sometimes unlikely things happens as the story develops.

As stated, Monster action cards are not intended for PCs. The Monster action card traits are intended to allow a GM to associate certain actions with certain NPC types quickly and easily.

Some monster action cards might be usable by PCs, especially the "NPC" action cards, but in general I would avoid allowing PCs from using Monster action cards. Most monster action cards represent specific physical attacks specific to a type of enemy. Things like Troll Vomit, Flame breath, etc just won't translate well.