Ok, let's start over again
When you play an item card from your hand (items cards are marked with a yellow light in the upper left corner), you put it in your play area, and it stays there (unless you discard it, which can be a volunteer act, or a consequence of using the item, such as per Large Rock). This card counts against your hand size of 7 cards, ok? When you play that item card from your hand, you take the marker and you put it on the card, not in your base.
Treasures are totally independant by all of this! When you pick up a treasure, you take the marker, you put it in your base, and you don't have to discard anything because it doesn't occupy "space" in your hand size.
When you decide to drop an item, you take the marker from the corresponding card and you put the marker in the square where you dropped it, and the card is removed from your play area and put near the board, away from all players.
A player picking up that object, simply spends one movement point while in the same square as the marker, picks up the card and adds it to his playing area, putting the marker again on the card.
Let me know if this is clearer