Getov, thank you very much for your valuable comments! Those are helping me to define the mechanics of items in my design, now I have a better idea of what to do.

And I have another idea flying around my head. I'm thinking when the player is about to sign out he will have the option to setup an automatic shop. That is when the player is out, his character will be shown camping in the last position he was, like a protected NPC, and his character will work there as a merchant. The gamer can choose which objects he wants to sell to other players that interact with his character. Do you think this might be a good idea?

Maybe you shoud limit the items the player can sell based on their progress (level/location). Lesser healing items and weapon repair at lower levels and greater items at higher levels or something of that nature.
That is a very interesting idea. May you could allow player that stop there to leave something with the player that they will find when they login again, like a note or gift.

You may also think of allowing bartering, trading items based on a price/value metric set by the player or a request list. "i have these items I don't need and I'm need this amount of X items."