Let's say my character killed a person before the roleplay took place, and he has said character's head. Do I need to have a substitute item for said head, or not ?

A Child's Toy would work. Slightly morbid use of the toy, but it has a head. I assume the head doesn't serve any purpose beyond being a trophy of sorts?

May be important to the story, but nothing magical, no.

On a somewhat unrelated note, the same applies for shrunken heads, true ? You know.. The ones that talk ?

Magical talking head, magical straw doll as a sub, I think.

I see.
Thanks !

Also what about stolen items ?
Let's say I stole a horn from somebody, I still have to buy it, true ?

That's correct! You stole the item, and that means you now own it, and you're using it in a post. Ergo, you would have to buy that item.

Alright.
What about backstory items ? If I no longer have them, do I still have to have them bought ? Or what if I buy an item then use it, is there a way to sell it ? Or buy the wrong item ?

If you're not using them in posts and just reference them as past possessions, you don't need to have them bought. Example, you say that your character had a ship long ago, but you've never used said ship in a post so you don't need to buy.

There's not currently a way to sell an item, but there will be in the future.

Not sure about buying the wrong item. Possibly contact an admin to get a refund? (pokes Jenna to come back)

I see. Thanks a lot !